Fund class codes are maintained in File Maintenance > Funds > Fund Code Maintenance > Fund Class.
To add a new one click the New button and enter in the new Code and Description. Next be sure to fill out the default natural accounts in the section below that will be used for the standard transactions.
Remember to also add the new Fund Class to the appropriate GL accounts in File Maintenance > General Ledger > Master Chart of Accounts Maintenance.
If you find during gift entry that the GL accounts are not populating correctly, check to make sure the right lines in the fund code maintenance screen were filled out properly.