Hi-
Daniel is the person for rat advising.
In terms of editing your ~/.afnirc file, if you have one already (does typing "ls ~/.afnirc", without quotes, produce an error for you on the command line?) you should be able to type
gedit ~/.afnirc
or
afni_open -e ~/.afnirc
to open the file in a text editor.
If you don't have it already, you can probably get the default one from the AFNI install (assuming you have followed setup locations from that) via:
cp $HOME/abin/AFNI.afnirc $HOME/.afnirc
... and then edit it.
--pt