You could always enter it as Cm7b5, or (from the user manual:
If you enter a lot of songs, you will appreciate these shortcut keys.
J = Maj7
H = m7b5 (H stands for Half diminished)
D = dim There is a setting in Display preferences to “Treat dim chord as a diminished triad instead of Cdim7.”
So, just enter CH and you'll get a half diminished chord.
Also, if you use The BeStPlain Chord Symbol font, there is an option as to how you want certain chords to appear. Look at around page 51 of the BIAB PDF user manual for more information (I say "around", because the copy I'm looking at isn't the most recent, so it may be off by a page or so).