I got this error after I was trying to export my map: Chimp found errors. View the 'chimperror' object and fix the corresponding faces.

So I clicked ok and then I got this:

http://s1273.beta.photobucket.com/user/GhostsOfWar666/media/photohelp_zps11324725.png.html

looks like you just need to seal the top of your map, then add a sky material
called +sky.

How do I seal the map and add the +sky?

your map is an open box, that box needs the top filled. sealed means no open areas.
you see them orange walls that go around the outside of the map,
you should select the vertices at the top of them walls and drag them down a bit, then click on the edge button, and select all the edges at the top of them orange walls and hold shift while you use the arrow to drag them up.
then to seal the top of the box connect all the top vertices by making polygons,
don't make them too big though.
next select all the new polygons including the ones that were created above them orange parts and hit M in gmax to open the material editor, choose a new sphere and name it +sky
click on the rectangle shaped icon beside the words diffuse and select a light blue colour that will represent the sky.