For the first person in trouble, try downloading pepakura 3.1.5 instead of the latest version. i also had a problem where it wouldn't print, but reverting to an earlier version fixed it. I have an epson printer running on windows 7.
As for the second person in trouble, printing on A5 should make no difference as long as you tell pepakura what paper to print on in print and paper set up, then make sure your the parts are all laid out properly and don't overlap. First put one piece onto a separate piece of paper then print just the one page to make sure its working just to be safe, theres nothing worse than wasting ink.