You will get the $8k refund about 2 months after you file. You must file a paper return and include the closing statement and other documents. If you're expecting a large return in addition to the $8k, you may want to file online and then subsequently "amend" your return to include the $8k FTHB rebate. That way you can get part of your money quicker. The $8k may slow down getting your refund.
Hope this helps!
If you already filed your 2009 Tax Return you can file an "amended" return and get your First Time Homebuyer $8k credit if you qualify. If you haven't filed yet, you can file it with your tax return. You have file a paper return and here's a link to more info: