Welcome to OcUK Forums. If you're trying to ask the shop, you should send them a webnote or post in the customer services forum at the bottom of the main forum page.
As for your question, it's almost certainly a Steam redeemable code, as DiRT 3 was almost entirely a digital release, with physical copies actually being few and far between iirc.
The G27 and DiRT 3 play excellently together. I know as I have both and it's the best rally experience on PC that I have had. RBR is still a better sim, but DiRT 3 is more fun. The G27 works very well with the game, but you may want to alter some settings, like reduce the degrees of rotation on the control panel to around 450 from 900. If you want any other tips I'll happily provide when I can. Either way, you're going to love the wheel!