can't see CTB files in Print Style drop down

I'm on a mac and have just installed Draftsight for the first time. Using the print configuration in the Sheet tab, I can only see the stb files and no ctb files. I have checked the folder, which contains both the  draftsight standard ctb and stb. Is there a global setting I need to use? I've been through all of the preferences, but can't see anything.

Any help appreciated Thanks