You found a bug.
You can fix the problem.
Edit tcl/ac3dprefs.tcl, find the line:
Code:
label_bool $wf prefs_output_rib_textures "Output RIB (Renderman) textures" "Output textures/UV coordinates in Renderman RIB files (untested)"
change 'prefs_output_rib_textures' to 'prefs_rib_textures'
AC3D should then recognise you want textures generated in rib files
Let me know how you get on.