Look in your User_Library_Application Support_Garmin directory for a "Custom Waypoint Folder" If it's not there, create it. Put your custom symbols in this folder and they'll be accessible to BaseCamp.
Don't understand why the Custom Waypoint Symbols folder is hidden away in the Library. Not really conducive to the average Mac user finding it and taking advantage of this feature.
By the way, when is BC going to recognize and send Custom Symbols to the Montana? :)
Thanks for the help! I was thinking icons instead of custom symbols, a location to put them, type of file, and finally a the file name. I found this all with your help.