Screwed unless support allows it to be reregistered. Or try and get your money back and buy through Impulse.

This is not Steam. When you finally get it to work you can happily delete Impulse if you wish. And it complete deletes itself. Unlike Steam. Plus you can still play online.

You must use the Impulse client to update your game.

No. You can play the game as comes on your disc just fine, without impulse or an internet connection, but updates (and thus multiplayer) require the serial number that is most likely missing if you bought it used (as the original owner registered it). I would try to get your money back if at all possible and buy it digitally or new. Otherwise its possible support (email [email protected]) may assist you, but they have no obligation to do so.

Edit: On a side not, you really should never by a PC game (and sooner or later console games) used, unless you've done a lot of research into whether they have systems like Steam or what Mass Effect 2 did that inhibits second hand sales.