Are you trying to mix the audio so that it removes the vocals from a song? Rush can't do that.
Are you trying to add titles representing lyrics of the song? If so, you can do it, but not automatically. You'll have to manually type in each lyric. And the program will not create a "cue" effect which shows you which lyric you are singing.
In fact, I'd say this is probably not the best program for creating Karaoke video. There are dedicated Karaoke videomakers available all over the web. My recommendation is that you use Rush for the kind of editing it was designed for -- otherwise you'll likely find yourself dealing with a lot of headaches.