Well showing the serial in a msgbox still involves a call to wsprintf, adding a string ("%d,0" or whatever), adding a buffer (25 dup(0) or whatever), and inserting the code.
That's about equally as complicated as adding a call to GetDlgItem/SetWindowText, which would consist of something like this:
(assuming that they're already imported, although importing them isn't much of a problem either

)
Code:
;either overwrite unimportant code or insert a jump to a cave, then add this
push nIDDlgItem ;the dlg id that you gave the edit box
push hDlg ;the hWnd of the dlg -- it'll most likely be in 1 of 2 places
;ebp + xx or in a buffer
call GetDlgItem ;hopefully it's imported already
push lpString ;since you've already got the serial in a buffer that you
;used with your MessageBox/Ex call

then just use that
push eax ;hWnd is in return of GetDlgItem
call SetWindowText ;hopefully it's imported already, if not try SetDlgItemText
That's more than enough info for you. You're selling yourself short though, by thinking that you can't do this already. You've already added the MessageBox code, so this isn't much more difficult if at all.
cheers,
will