If you’re part of the OS X Beta Program, you can click this link to sign up to receive beta versions of SpamSieve via the built-in Software Update command. There’s also additional information here about pre-release versions of Mac OS X. You can click this link to stop receiving beta updates.
If you encounter any problems using a beta version of SpamSieve or a beta version of Mac OS X, please report them via e-mail rather than in the forum.