i need to program a dxp to have account codes for nite. i have never did it. i think i have to go into cos and change some things. any help would be great thanks for the help

You are on the right track. Pick a new class of service for the night mode. Copy an existing COS then turn on forced account codes. Go to your stations and give the night COS that you created there. Don't forget to create new account codes.

can i have just one code for the whole system or do i have to have one code for each ext? thanks

You can have just one. Account codes are not tied to specific extensions. Authorization codes are. You should be ok using just one.