If you don't want to be troubled by downloading and installing source you can do this:
1) as root type: perl -MCPAN -e shell
2) You'll need to pass through some configuration... most of it may be default, you'll need to choose the closest mirrors to you, but that's easy there.
3)type: install Mail::Spamassassin
it might ask you about dependencies and you just need to tell it to grab them too... go to http://www.spamassassing.org
and try to install the pre-requisites first - just to be on the safe side...
(pre requisites installed in the same way)