can't change fonts

I'm on a mac running 10.5.6 running mged 7.13.0.
Trying to use the Fonts command under File Preferences
I get a box with an error msg:
Error:named font "button_font" doesn't exist

when I click the Ok or Skip Messages buttons, I get a small, but empty window, titled font_scheme . The details button gives me:
named font "button_font" doesn't exist
named font "button_font" doesn't exist
while executing
"font configure $fname"
(procedure "font_scheme_init" line 45)
invoked from within
"font_scheme_init id_0"
invoked from within
".topid_0.#topid_0#menubar.#topid_0#menubar##id_0#menubar#file.#topid_0#menubar##id_0#menubar#file##id_0#menubar#file#pref invoke active"
("uplevel" body line 1)
invoked from within
"uplevel #0 [list $w invoke active]"
(procedure "cad_MenuInvoke" line 33)
invoked from within
"cad_MenuInvoke .topid_0.#topid_0#menubar.#topid_0#menubar##id_0#menubar#file.#topid_0#menubar##id_0#menubar#file##id_0#menubar#file#pref 1"
(command bound to event)

I'd really like to be able to use a larger font in my command window (my eyes aren't what they used to be).

Thank you