Once your Google Voice account is setup, you need to sign up for OBiTalk at www.obitalk.com. On the right column, click "Create a new account". You may sign in directly with your Google account by clicking "Sign in with Google".
Alternatively, you can sign up by filling in the lower registration fields with your email address (this does not have to match the Gmail address you used for your Google voice), password, and the name which will be displayed for your OBiTalk account. Then click "Continue".