Hello,
As Pinpanther56 stated, the clients must have the latest windows update software installed.
If correctly working, the clients will obtain the latest version via the selfupdate site within WSUS.
If they have not obtained the latest windows update files then they wont appear in the computers section.
The most common problem is permissions on the WSUS website and permissions on the virtual directories. Eg. Selfupdate.
Check the windowsupdate.log file on one of the troubled computers c:/windows/windowsupdate.log, if you can see HTTP401 error then you have permissions errors.
Check the permissions on the website and virtual directory's. Anon login should be flagged, however I had to flag Windows Authentication aswell for it to work.
If you still have problems, and your WSUS resides in the default website on port 80 then uninstall WSUS and reinstall WSUS in it own webiste. Make sure you change your GP to reflect the port your WSUS resides on.
I took me several days, I must have encountered every problem under the sun, and eventually got it working.
The most helpfull tip is to read the windowsupdate log file.