No, you don't need a forum account, just one on the Larian Vault. Log in there, or create a new account, link it to your kickstarter/paypal email if you have not done so yet (click on the kickstarter logo), and select where you want the digital version(s) of the game, etc.

The kickstarter backer code should be in a red box on the Summary page (shown by default after clicking on 'D:OS Kickstarter').