If you're using the newest 6.11.4, then here it is:

Code:
  
sky cube
{
 type = <level2sky+6.tga>;
 flags = cube,visible;
 material = mat_sky;
}


The TGA can be any name, but it needs the "+6" on the end and it needs to be a 32-bit TGA with an all white alpha channel...if it isn't then the sky texture will blend in witht the cube texture(I don't know if this is actually required or not..I've been using cubes since before they became a part of the engine and that's how it was done). Don't forget the material line, either. Anywhere a standard sky texture appears(like on a skybox), this will appear. I have no idea if a skybox is not required, but just know that I do and have no problems.

My User Contributions master list - my initial post links are down but scroll down page to find list to active links