Please refer to the master Pay Codes link for more information.
Note that Pay Codes are set in two places on the Project: on the Time tab to determine a default and in the left menu to determine which list of Pay Codes is allowed on this project.
Note that if no Pay Codes are specified in the left menu, then all are available.
Navigate to Project > List > List tab > Edit Pencil Icon > Time > Default Pay Code.
Navigate to Project > List > List tab > Edit Pencil Icon > Pay Codes.