Tap Tap Revenge 2 provides a selection of songs for you to download free. You can't really "search" for songs, you can only have the ones they offer (there's lots, there's really something for everyone.) To download songs for TTR2, you will need a Wi-Fi connection (if you don't have one at home, you could go to free Wi-Fi hot-spots outside like Starbucks or McDonald's.) Launch TTR2, and on the left of the big red play button it should say "free tracks". Press that and then go to "new music", where you will be able to choose from easy, medium, hard, extreme, top rated, themed tracks and more.
I hope that's what you meant. If you meant how can you choose songs that aren't in the TTR library, (I.e, songs from your iTunes library), I don't think that's possible yet. But Tap Tap Revenge 3 is coming out soon, and apparently that will let you access your iTunes library.