(moved to X-Plane discussion)
AC3D is a lot more free and easy when displaying textures. Some things need to be correct for textures to appear in X-Plane
from
http://www.inivis.com/forum/showthread.php?t=7174
1. file names do not accept spaces: housewall.format
2. formats must be .png or .dds
3. side measures of paint file must be equal, e.g. 512x512
4. Object must be 1.
Actually I believe 3) should be "paint file sizes must be a power of 2 e.g. 512x1024. Correct me if this is not the case.
The location of the file is also important, for X-Plane to pick up the file.