Simple Question?

I would like to know how I can score some music to an iMovie project I am just finishing.

I started to create something in GB, but I stopped when I could not figure out how to work in time that I can relate to my film, ie clock time instead of music time.

Does this make sense? The example is this:

I have an edit that I want ot have a musical cue on. I know that edit comes at 13:06 seconds, how do I make sure the musical cue in GB starts at that moment?

I would have thought this would be more obvious given the "iLife" pitch, but I can't "see" the answer.

TIA

Henry