Maybe the sky map is too low to see. Try adding a negative tilt to the sky map and adding the overlay flag:


sky cube5
{
type=<skycube5+6.tga>;
flags=cube,visible,overlay;
tilt=-10;
layer=2;
z=0;
}


I don't claim to be an expert, so I don't know what the z means, but that's how mine works. I hope that helps.