You can run into problems when you have both Screenwriter 6 and Screenwriter 6.5 installed on the same machine. Each version has a different activation system that takes a different kind of serial number.
DO YOU HAVE THE RIGHT SERIAL NUMBER?
When you receive the email with the serial number for Screenwriter 6.5, there is often a second serial number for Screenwriter 6, for use with the Windows version of Screenwriter. You need to make sure that you are using the CORRECT serial number to activating the desired version of Screenwriter:
Screenwriter 6 requires a serial number that begins with "SCRW0600"
Screenwriter 6.5 requires a serial number that begins with "MMSW"
So if you are trying to activate Screenwriter 6.5 with a serial number that begins with "SCRW0600", you will likely get an error message that says:
Part of the serial number you typed is missing or incorrect.
Please make edits and press Verify when done.
ARE YOU LAUNCHING THE CORRECT VERSION OF SCREENWRITER?
You might also be launching and trying to activate the WRONG version of Screenwriter. If you are trying to activate Screenwriter 6.5, and your internet browser brings you to an activation page, then you are actually launching Screenwriter 6 (or 6.3). Screenwriter 6.5's activation does not launch your browser, it starts a special application called "Activate and Register Screenwriter".
PLEASE look at the Screenwriter app in the Finder, and do FILE > Get Info on the application file to determine which program you are trying to run / activate:
Screenwriter 6 is located in the /Applications/Screenwriter 6 folder
Screenwriter 6.5 is located in the /Applications/Screenwriter 6.5 folder
AFTER UPGRADING TO 6.5, DELETE THE OLD SCREENWRITER 6 FOLDER
If you have upgraded and activated Screenwriter 6.5, there is no reason to keep the old Screenwriter 6 on your system. You should DELETE your old /Applications/Screenwriter 6 folder by dragging it into the trash. There is no danger to doing this, because (usually) your scripts are NEVER permitted to be saved in the actual application folder.
INSTRUCTIONS FOR DOWNLOADING, INSTALLING, AND ACTIVATING SCREENWRITER 6.5