So. I was editing a skin on skindex but I can't get between the arms.
Not sure what you mean by between the arms, but I thought you meant the inside so here's my tutorial if that's the case;
Remove all the other body parts except for the arms. Then drag the skin with your cursor so you can see the inside of the arms, then you can paint inside there.
I am most active on Discussion, but you'll see my messages all around the place since threads get moved a lot.