Need HELP!

You can't enter for DLC unless steamgifts knows you have the base game.

System would recognize you have the game since you didn't have it when you synced your account last time, thus enabling you to enter Magicka DLC giveaway.

It would confirm that you have Magicka and let you enter for DLC for it.

Syncing SteamGifts with your Steam Account, recounts the games and groups you have. Else it won't know about the changes. They automatically do it every so often but if you want to speed it up, do it manually.