Enabling credit at the advertiser level can be done within the advertiser's window.
From the Main Menu, Go to Advertisers -> Advertisers. Click on the Advertiser's name to open the Advertiser Window.
From the Advertiser's window toolbar. Click on Enable Credit...
You will need to confirm Enabling Credit at the Advertiser Level.
When you confirm enabling credit at the advertiser level. The advertiser's credit will be shown right next to the advertiser's name.
Enabling Credit at the advertiser level won't affect any contracts. You will still need to enable credit at the contract level to use the advertiser credit.