@Mark-Wadsworth Are you sure you bought the Mac version of Sound Forge? If you got a Windows version it would explain why you got an exe file and your serial number doesn't work.
@Mark-Wadsworth Please check your order confirmation e-mail. If you bought the Windows version you need to contact sales support at infoservice@magix.net or shop@magix.net.
@Mark-Wadsworth We are just users like you. Here is no Magix staff. I don't know what you are doing and why it's not working. Your descriptions are not detailed enough so I'm mostly guessing.
SP, sorry if I was a bit rude. You have been very helpful and thanks for all your help.
The Magic site didn't ask me to choose "Mac" or "Windows" so I assumed it would automatically download the correct one, in other words the "Mac" version for me.
I think we understand your frustration but, as @SP. says, we are all fellow-users, not Magix staff. ".exe" files are Windows only, the Mac version would be a ".dmg" file
Did you also note what @SP. wrote about Sound Forge Mac not being able to run under any recent release of Mac OS? This pinned post explains the official situation in this regard.
If your Mac runs a recent version of the OS you need to immediately request a refund. Email infoservice@magix.net and explain what has happened.