Greetings,
A month or so back I must've wasted a solid day or two trying to figure out why half of skyboxes I had downloaded didn't work and why my own custom skyboxes weren't working. Today, I revisted the problem and found a solution.
Your mileage my vary, this may not work for you, there may be better ways to do this, etc. This is my experience:
First of all...your image files cannot be .tga. Only dds, jpg, and bmp work for me.
Second...the .x file MUST be three letters and all CAPS. For example, SKY.x is fine; sky.x or Sky.x or sKy.x etc will not work.
How to make skyboxes with TGA images work in FPSCx10:
1)If you have a bunch of skyboxes that you downloaded which use .tga, you must first take all the images and re-export them to jpg (or dds or bmp).
2) Then, open up the .x file in wordpad and replace every instance of "TGA" with "JPG" (or "DDS" or "BMP"). There will be 6 instances.
3) Save the file and rename it using three letters in all CAPS.
4) Run FPSCx10, scroll until you find your skybox.
Hope this is helpful to someone and saves them some time.
Cheers,
Darin