Looked good, installed ok. Ran, registered email, told me to enter serial number. What serial number? tried everything I could think of to find one. Nope. Just kept running me around in circles. Uninstalled. Better luck next life.
Did you download from FxHome of from a third party site? You should have been emailed a password. Check your Spam Folder.
Or, open the menu top right of this page and click account. That should take you to a management page where you can retrieve your information
I am assuming your email used for this profile is the same email you used to download the installer. Your software is tied to the account used to download it.
Or, try contacting support: http://hitfilm.com/questions/submit
Got me running in circles too! The trick is to register (i.e. creat a free account) BEFORE dowloading. It seems once you've registered, "they" prepare a file with your ID somehow encoded in it.
Anyhow: if you've already installed it (as I did) THEN created your account, and clicked on a link that brought you to a download page again... Well, download it again! And install it. And there you go. It's odd, but quick enough.
There is probably a more straight-forward way (like skipping the first dowload, creating an account, signing in and downloading from that session...), but I didn't find when as was, as you were, getting crazy
Where did you get the installer from?
Typically people find Express via the Express page, where you create an account (or log in) and get a license assigned to your account at the same time. This happens before you receive the installer, so there's no issue.
If you install the program before creating an account/claiming a license, you will need to do that. You can use the same page, just fill in your details and submit, the license will then be added to your account. You don't need to re-download the software, just Activate it.