I've got no idea about the level requirement, though I never heard one way or the other. I am currently level 27 though, so that shouldn't be the issue. I'll go walk around in Whiterun a little while, maybe it is just something that comes up after a bit.
Just restart your 360. I was having the same problem but you just need to restart it so the game can load Dawnguard. --- "I KNOW what metaphor means. I'm in 10th ****ing grade, all honors classes" -RageAganstAudio
If you are under level 10 though, then don't even worry about DLC. There are tons of adventures and lots of fun to be had in Skyrim for you before you need to worry about Dawnguard. Heck, I've played 70 hours and still have an untold amount of game that I could play. Plenty have played waaaaaaay more than me and maxed out and everything.
Its ot a requirement to be level 10. Bethesda just said that it was recommended to be past that level before beginning. I believe I read that in the game informer article on dawngaurd. --- gamertag:kozmo81
Vampire attacks occurred various times (twice actually). And no place would so much as mention the Dawnguard reforming. But once I hit lvl 10, I talked to a guard and the first thing they said was about Dawnguard.
Sooo, Going to take a guess and say its required, as my Vamp was lvl 8 when they said nothing about Dawnguard.