Alternatively, you can do this manually. Hold down the Control key and click on the SpamSieve icon in the Finder. Choose Show Package Contents from the menu. Open the Contents folder, and then open the Info.plist file using TextEdit. At the bottom of Info.plist, change:
<key>LSUIElement</key>
<string>0</string>
to:
<key>LSUIElement</key>
<string>1</string>
Now with 2.8.5 I do not see this in the info.plist
SpamSieve has not changed, and I don’t think 10.6.6 did either; my guess is that the difference is with the Launch Services database on your Mac.
In the past, people have run into problems using Property List Editor to do this. That’s one reason that the instructions specifically say to use TextEdit.
Which application are you using to edit it, and what is the exact error message that you receive? If you don’t have privileges for editing the SpamSieve files, you could install a fresh copy of it in your home folder or use an application such as TextWrangler (not the Mac App Store version) that can edit protected files.