Every time I try to play 18 wos convoy I always get a box that says prism3d.exe not found can anybody help me I want to play this game?

Try downloading and installing updated drivers for your video card. If you have an nvidia card, you can get them from NVIDIA Drivers
If you have an ATI card, you can get them from Drivers & Software
If you have any other type of card, you can post here and we will try to find a link to some drivers for you, or you can try the webpage of the manufacturer.

mine says 18 wos convoy with a red X before it