1) Unzip the custom map .pk3 file to your custom folder directory. This is the default location for custom .pk3 files:

Allied Assault: C:\Program Files\EA GAMES\MOHAA\main
Spearhead: C:\Program Files\EA GAMES\MOHAA\mainta

So, just place the custom map .pk3 file inside the \main or \mainta folder (depending on what game the mod is for) and you're all done. If you want to delete the .pk3 file, just go into the folder, click the icon, and press delete.