Here's the code for a sky cube:

STRING* test_wmb = "test.wmb"; // put your own level name here

void main()
{
fps_max = 70;
video_mode = 7; // run in 800x600 pixels
video_depth = 32; // 32 bit mode
video_screen = 1; // start in full screen mode
level_load (test_wmb);
wait (3);
ent_createlayer("skycube+6.tga", SKY | CUBE | VISIBLE , 1);
}