How to give a field a default value

[ZiNgA BuRgA]

Okay, I think I might be understanding you a bit more...
Is it the case that you already have a bunch of threads, but don't have this data set in the thread fields (ie you installed XThreads (or created the thread fields) after the threads were posted)?  If so, then you'd want to enter the data into all the thread fields, either manually or using a script (that just fills the database rows).  Doing anything to editpost makes no sense here.

Otherwise the Default Value will not appear on editpost, since it ultimately is just a default that gets entered when creating a thread.  When you've already got a thread, the value has been explicitly set by the thread, so a default makes no sense.