Enabling credit at the Contract Level can be done in the Contract Window.
From the Main Menu, Go to Advertisers -> Contracts -> Contracts
Click on the Contracts name to open the Contract Window.
From the Contract's window toolbar. Click on Enable Credit...
You have the option to either have the Contract use its own Credit or use the Advertiser Credit.
Click Save & Continue. The Contract Credit will be shown right next to the Contract Name.
Make sure you add Credit to the Contract by going to Contract Credit -> Adjust Credit...