From a MSKB article here:

http://support.microsoft.com/kb/914968

1. Start Money.

2. On the Banking tab, click Account List.

3. In the Account list, right-click the credit card account that you want to use, and then click See Account Settings.

4. In the Account type list, click Bank.

5. In the Balance Information section, type the credit card limit as a negative number. For example, if the limit for the credit card is $1,000.00, type -1000.

6. In the Account type list, click Credit Card.

7. In the Change account settings dialog box, click Done.