Thank you for your response! Unfortunately I'm still having issues. My CustomAtlases.niml file has my rat space and I set the AFNI environment variable AFNI_SUPP_ATLAS_DIR to the right place but my atlas apparently doesn't contain anything. When I run the command "whereami -show_atlas_code | less" I get this happening for the atlas I made.
Atlas Rat, 0 regions
----------- Begin regions for Rat atlas-----------
----------- End regions for Rat atlas --------------
Any ideas for what's going wrong?