Using older mountain bike in a Trainer I have an older mountain bike.  I need to change the rear skewer so I can use an indoor trainer, but I don't think there is one.  What do I do?

I have a CycleOps Mag trainer and they included a skewer to replace the one that is supposed to be on the bike.
The point of the quick release skewer they give you with a trainer is generally to prevent damage to your fancy skewer already on your bike, and make sure the trainer can grip onto the axle properly. If you have a quick release system, you should use the provided skewer. 

If you have a thru axle like a Maxle, you may need to buy a special adapter. 

If you have a classic nutted axle (which you most likely have), you can generally use it directly in the trainer, but you may want to double check with the trainer manufacturer.