Download and install MC 10 from the link near the top of the forum.
Restore your license using your saved license code. A key will be generated that is good for 14 days, save it somewhere outside of MC10 (My Documents) after the automatic restore is finished.
Be sure you use I.E. and also the same email address you used when purchasing MC. If your email has changed you will have to post again to have J Riverchange your email. DO NOT publish yourMC registration code.
Once you have MC10 reinstalled and registered, you can purchase the upgrade anytime and get the upgrade price (10$US at the moment). MC 10 & 11 can co-exist on the same computer, just put MC11 in a different directory, (ie: Program Files\J River MC 11\Media Center 11).
I would recomend using MC10 at first and do all your tagging and other changes there. Then do a library backup, close MC 10, open MC 11 and do a library restore from the MC 10 backup file. MC 11 has a different database structure and is not backwards compatable, (you can't use data from MC 11 in MC 10). So, untill you're comfortable with MC 11, I would recomend you proceed as described. Just my humble opinion.
I'm afraid I can't help you with you iTunes other than to suggest you check the portables section of the forum (
http://yabb.jriver.com/interact/index.php?board=8.0), there's been alot of iTunes disscusions there.
Good Luck and welcome back!