I'm having trouble buying the land in Falkreath. I've already received the letter from Siddgeir but he won't show the dialog option for purchasing land. What am I missing?
You have to talk to the steward if you want this house. If it still doesn't have the option, you might have:
Killed Helvard for the Dark Brotherhood
this will make Siddgeir's disposition low and you won't be able to buy it since he hates you. The only way to fix this is side with the Stormcloaks in the Civil War and complete the questline so the Jarl and Steward will be replaced and you can assist the people and then talk to them about the house.