Subversion has no built-in mechanism for searching the contents of a repository. You can search the contents of your working copy using any desktop tools at your disposal (like Spotlight or grep).
When you say "repository" make sure you don't really mean working copy.
There are third-party products that can provide a (usually web-based) search engine for a Subversion repository. The only one that I know of offhand (but I'm certain there are others) is FishEye but I haven't used it myself.