To find the serial number of the program you just downloaded, look at the top right corner in the Register window that appears when you run WinMLS in unregistered mode (it is not enough to install, you have to run WinMLS also).

If you have not yet installed and run WinMLS, follow this procedure:
  1.  the full version (no limitations) of WinMLS for FREE (~8MB). You can use it for 30 days.
  2. Install the software.
  3. Run the software (found here):