The dash is not needed to enter the code. You should be in your main game menu when you select downloadable contents. After that, just enter the code(without the dashes) that came with your original xbox CD. The server should recognize this code and display a 'Purchased' note at the bottom of the DLC. With this, select the content again, and you should now have the download option available to you.

DA is so different from MW2, whether or not you prefer RPG or FPS gameplays. Imo, DA is the best game i've played in a long time.