Have you messed with the rock's .mat? (If you have one for your mesh) Make sure the texture is directed to the picture file inside the same folder, otherwise it won't work.
For example, the original rock has a mesh file, a picture file, and a material file. (.mesh, .png OR .jpng, and .mat)
Open the .mat with notepad and should see something similar to this:
material rockMat1
{
technique
{
pass
{
cull_hardware none
cull_software none
texture_unit
{
texture rockTex1.jpg
}
}
}
}
If you wanted to just retexture the object, make a texture, delete the current rockTex1, and rename the new texture rockText1.
If that doesn't work, save all presets, maps, etc elsewhere and reinstall FH. ^^