Did anything change to.start this problem?
First I would try clearing the data from the messaging app.
If nothing, try a cache, davlik, and fix permissions with Cwm.
If nothing again, I would grab an MMS.apk from a downloaded cm10 zip. And paste it in the /system /app, and set the permissions to rw-r-r.
Again if nothing, maybe something got tweaked in /system or the settings. I would dirty flash the latest cm10 nightly in cwm with only wiping cache and davlik cache. Make a backup first as always.