Call of Duty - Adding a map to your server.

Login to your server via ftp ( Tutorial Here ).

Locate your servers main call of duty folder.
Change to the folder main.
Upload your maps here and restart your server.
Change to your map by typing map <mapname>

A few notes.

Make sure the map you are uploading is for call of duty game.
If your map won't work it may be corrupt. Download it again and then re-upload it.