I don't have this pack you are working with. I assume when you import your Guitar it is already attached to the part of the body it should be? Surely this is the case if it is a bought pack. If not you may want to check that. If it is atteched to the correct part I think its just a case of lining it up but some animations may make it look as though its going everywhere. You are best just to put a pose on it Saying that the pack should have something like this yes?
As for the hands I am sure you know this but just in case!!! Go to the timeline and click on hand. Your hand modifier will appear and a you will be able make it look better I suppose like the hands wrapped around the fret!
Hope this helps. If not forget LOL