You need to construct an "entity" to make your door. The door is usually a "map entity" though you can use a model. You have to build your door separately and then compile it as a map entity. You have to set up your door properly so that it will pivot around the proper axis. You import your door into your level, position it and then assign it an action. The action is code that you either write yourself or use the template codes. If you want a sliding door you assign it a "lift" code.
The wed tutorial gives you more detail. You can get it on the downloads page.
Also it is not always the best idea to use CGS subtract to make doors. It is to easy to cut into floors and ceilings. Construct your wall with blocks and leave an opening for the door. CGS is fun to use... I use it to cut out windows.
Good luck