After purchasing a membership, I still can't move the experience slider in the alternative advancements window. It's stuck at 100, and now my character can't level.
Did you buy membership while logged in? If so, you need to camp to desktop, and then log back in. If not, are you using a non-standard UI?
Membership will allow you do adjust slider, if purchased membership while logged in, restart the game. If that doesn't work, what level are you? Does your current expansion allow a level increase such as the latest expansion allows levelling to 125.
Oh, you're right. I didn't even see the 'now I can't level' part. If you only went from free to play to member, but haven't bought the expansions, you'll still be locked at level.. 100?.. with the AoM expansion.