Now, you need to make sure that your copy of Dance Central (note, not DC 2) is first-hand and you will also need Internet connection. On the back of the DC 1 manual that comes with the disc, there should be a redeem code (I will have to confirm that when I get home tonight). This is one time use only (hence the first-hand prerequisite). If it's second-hand, it's probably not a problem as long as the previous owner hasn't already used the code.
The songs will still cost 400msp on top of the redeem code; however, if you preordered DC 2, you should be able to get it for free through a voucher code (I am not too sure as I didn't preorder mine).
Now load up Dance Central 2. On the main screen, select Options->Redeem code. Enter the code from the first paragraph and confirm the purchase. It will start downloading the Dance Central Import Pack. Hope this helps.