Update
I had re-installed it previously. Just exited Apple Mail, started SpamSieve and installed the Apple Mail plugin.
No errors in the system error log directly related to SpamSieve:
12/1/14 3:57:21.000 PM kernel[0]: Sandbox: softwareupdated(590) deny mach-lookup com.apple.webinspector
12/1/14 3:57:21.542 PM nsurlstoraged[80295]: purgeEntireFileSystemStore - failed to rename the current cache dir /Volumes/craig/Library/Caches/com.apple.Safari/fsCachedData to new cache dir /var/folders/z3/t37gf4g91bj568_0763f25fr0000gq/T/fsCachedData_remove. Errno=18
12/1/14 3:57:29.510 PM nsurlstoraged[80295]: purgeEntireFileSystemStore - failed to rename the current cache dir /Volumes/craig/Library/Caches/com.apple.Safari/fsCachedData to new cache dir /var/folders/z3/t37gf4g91bj568_0763f25fr0000gq/T/fsCachedData_remove. Errno=18
12/1/14 3:57:35.931 PM nsurlstoraged[80295]: purgeEntireFileSystemStore - failed to rename the current cache dir /Volumes/craig/Library/Caches/com.apple.Safari/fsCachedData to new cache dir /var/folders/z3/t37gf4g91bj568_0763f25fr0000gq/T/fsCachedData_remove. Errno=18
12/1/14 3:57:37.749 PM discoveryd[63]: Basic Sockets SetDelegatePID() failed for PID[3950] errno[3] result-1]
12/1/14 3:57:38.591 PM nsurlstoraged[80295]: purgeEntireFileSystemStore - failed to rename the current cache dir /Volumes/craig/Library/Caches/com.apple.Safari/fsCachedData to new cache dir /var/folders/z3/t37gf4g91bj568_0763f25fr0000gq/T/fsCachedData_remove. Errno=18
12/1/14 3:57:55.848 PM discoveryd[63]: Basic Sockets SetDelegatePID() failed for PID[3950] errno[3] result-1]
12/1/14 3:58:12.854 PM com.apple.xpc.launchd[1]: (com.apple.xpc.launchd.user.503.100023.Aqua) Caller specified a plist with bad ownership/permissions: path = /Volumes/craig/Library/LaunchAgents/com.c-command.SpamSieve.LaunchAgent.plist, caller = launchctl.64918
12/1/14 3:58:18.000 PM kernel[0]: Google Chrome He (map: 0xffffff80c564c000) triggered DYLD shared region unnest for map: 0xffffff80c564c000, region 0x7fff90e00000->0x7fff91000000. While not abnormal for debuggers, this increases system memory footprint until the target exits.
12/1/14 3:58:28.194 PM discoveryd[63]: Basic Sockets SetDelegatePID() failed for PID[3950] errno[3] result-1]
12/1/14 3:58:42.366 PM lsregister[64948]: LaunchServices: Begin database seeding
12/1/14 3:58:42.367 PM lsregister[64948]: LaunchServices: Completed database seeding
12/1/14 3:58:42.369 PM lsregister[64947]: LaunchServices: Begin database seeding
12/1/14 3:58:42.369 PM lsregister[64947]: LaunchServices: Completed database seeding
12/1/14 3:59:14.888 PM discoveryd[63]: Basic Sockets SetDelegatePID() failed for PID[3950] errno[3] result-1]
12/1/14 3:59:26.956 PM com.apple.xpc.launchd[1]: (com.apple.quicklook[64980]) Endpoint has been activated through legacy launch(3) APIs. Please switch to XPC or bootstrap_check_in(): com.apple.quicklook
12/1/14 3:59:33.079 PM discoveryd[63]: Basic Sockets SetDelegatePID() failed for PID[3950] errno[3] result-1]
12/1/14 3:59:46.098 PM Preview[88445]: No items found in keychain
12/1/14 3:59:46.102 PM Preview[88445]: No items found in keychain
12/1/14 3:59:46.257 PM WindowServer[80052]: WSGetSurfaceInWindow : Invalid surface -1627300343 for window 4798
12/1/14 3:59:49.000 PM kernel[0]: Google Chrome He (map: 0xffffff80a7bc8690) triggered DYLD shared region unnest for map: 0xffffff80a7bc8690, region 0x7fff90e00000->0x7fff91000000. While not abnormal for debuggers, this increases system memory footprint until the target exits.
Looks like it took. Everything is world readable and writable by my userid.
$ ls -R Library/Mail/Bundles/SpamSieve.mailbundle/Contents/
Info.plist MacOS Resources _CodeSignature
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//MacOS:
SpamSieve
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources:
Danish.lproj Korean.lproj MavericksChangeSettings.scpt Russian.lproj
Dutch.lproj LeopardChangeSettings.scpt MavericksTrainGood.scpt Swedish.lproj
English.lproj LeopardTrainGood.scpt MavericksTrainSpam.scpt Vietnamese.lproj
French.lproj LeopardTrainSpam.scpt MountainLionChangeSettings.scpt YosemiteChangeSettings.scpt
German.lproj LionChangeSettings.scpt MountainLionTrainGood.scpt YosemiteTrainGood.scpt
Italian.lproj LionTrainGood.scpt MountainLionTrainSpam.scpt YosemiteTrainSpam.scpt
Japanese.lproj LionTrainSpam.scpt OpenWindow.applescript scriptrunner
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Danish.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Dutch.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/English.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/French.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/German.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Italian.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Japanese.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Korean.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Russian.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Swedish.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//Resources/Vietnamese.lproj:
Localizable.strings
Library/Mail/Bundles/SpamSieve.mailbundle/Contents//_CodeSignature:
CodeResources