AdminCP -> User Profile Fields -> Add New User Profile Field -> Single-Line Text Box -> Fill in the form with whatever you want -> Save
Note the internal name of the field (not what you called it, but what vB is calling it). Example: "field9"
Now go to Styles & Templates -> Style Manager -> Your Theme -> Edit Templates -> Postbit Templates -> postbit/postbit_legacy (depending on what format you're using)
Then just add this in in the same area as registration date and such. You'll have to tweak it to match your theme and use the correct field ID.