I have bought and downloaded the Hearthfire DLC and I am the Thane of Falkreath and I am still not able to buy land for my house yet.

I think I was also already the thane in Falkreath, but it was over a year ago so can't be sure. I talked to Sidgheir, I think I asked him if he needed help, he sent me for the mead which was definitely the second time he ever asked that of me, I bring it back, he says I have to be known among his people first so I exit the conversation, count to 3, reinitiate and the quest pops up that I completed 3 of 3 favors, and then he declares me thane again. Then go speak to the steward. So even if you're already thank, you have to repeat some steps.