There really, really needs to be some sort of in-game notification when people have reached level 13 to let them know to continue leveling up they need to buy the first map pack. I see way too many people, here, on other forums and especially in-game who ask about why they can't level up, and it seems to be a very serious issue because those who don't figure it out will probably think something is wrong and delete the game. I've seen plenty who have given the game a bad rating on the iTunes store too because they think it's a bug or something wrong with the game.

It should definitely not pop up right as you hit level 13 since you might be in the heat of battle, but in some way people need to be told very clearly that they need the first map pack to continue.