Invisible entities is one frequent question in the forum, it is often the same problem:

Press F11 twice, when you see the blue box around your entities, you know they are there.

If they are there, then open them in MED and check if they are just totally transparent. You can set them transparent two ways, either by using a transparent skin texture with alpha channel, or a transparent material. You will see then them in the windows in MAX and in MED because transparency is not used there, but you wont see them in a real game.

If that is so then give them a not transparent texture, and also remove the totally transparent material, and you now can see them.

Hope this helps.