I've loaded Audacity and am learning how to use it. So far, it's been great, but I'm stuck on one thing: is there any way I can add a beat tone (like a metronome) to a selection of music, either manually or automatically? (from someone who needs all the help she can get to stay in time... )

In the Generate pull down there is an option called Click Track.
I think this will be what you want.

(I use the Linux version so if its laid out differently I apologise - but a "Click Track" is what you're looking for.)