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