Hey Andy, this link has a howto for setting up compiz. It is actually for Xubuntu Gutsy, and I don't know whether you are using the Cassandra stable or the Daryna beta 8. If you are using the beta, it should all work, but since you already have it installed, that should not be a huge deal. You can skip down to the part where he talks about making compiz default. The first method he describes I know works in Mint, as I have done it before. The "more-difficult-but-better" way I have not tried, so I cannot say for sure. However, I think it would work, since most (almost all) *buntu tutorials will work in Mint also. Hope this helps you out.
http://xubuntublog.wordpress.com/2007/12/09/xubuntu-compiz-pretty-pretty-xubuntu/