You have to go through your account to activate the slots. It was done this way because the idea was for you to pay for each slot. When navigating your account via the website you'll see an option to enable them.
Since the game is currently free you can use the max amount, but remember that if/when SE decides to charge that just deleting your character isn't enough. You would have to go back to your account and deactivate the slot. I'm almost certain that they can be deactivated regardless of whether or not you have a character assigned to them. Similar to XI, these character slots act as a content ID where you pay to have them regardless of whether you use them or not. Hopefully this is helpful.
Edited, Jul 23rd 2011 7:56pm by FilthMcNasty
Only hips + boobs all day and hips + boobs all over my icecream
30 bucks is almost free
Its personal preference and all, but yes we need to educate WoW players that this is OUR game, these are Characters and not Toons. Time to beat that into them one at a time.