I suggest that somebody of you or even better all of you report this problem by email to IDM support, see top of this page. IDM is perhaps not be informed about this compatibility issue with MAC OS 10.10. This is a user-to-user forum as stated at top of this page.
I'm using only UE for Windows. The font settings are stored in the sections [Font] - command Set Font - and [FixedFont] - command Set HEX/Column Mode Font - in uedit32.ini on Windows. The structure of uex.conf is most likely different, but searching not case-sensitive for font should not return too many lines containing this string for identifying the right ones for document window area.