Hello.
To remove the watermark from Windows 7 beta builds, you must use a resource editor to 'patch' a system file. I am assuming that since you don't know what file to edit to get it back, you used a third party patcher
Luckily, most patchers make a backup of the system file edited.
First look for a backup in %systemdrive%\User32Backup\ (ex. C:\User32Backup\)
You should be looking for 'user32.dll.mui'
If that folder does not exist check in %windir%\System32\en-US\ (ex. C:\WINDOWS\System32\en-US\)
You should be looking for 'user32.dll.mui.bak'
If you find either of those files, simply replace the 'user32.dll.mui' file that is currently in %windir%\System32\en-US\ with one of the backup copies, restart, and report back here.
Thanks and good luck.