I've just come off the phone with Adobe support after trying in vain to activate a trial version of Flash Builder 4.7 Premium with a serial number.
No matter what, I was always presented with a prompt that asked me to 'renew my subscription', referring to a Creative Cloud subscription which I do not have nor need.
What I do have is a valid serial number. I tried quitting the Creative Cloud app and relaunched FB 4.7: no joy. I chatted and phone Adobe Support: no joy. They told me to email them, and finally someone called me and fixed the issue. Here's how.

Open your Creative Cloud desktop app. On a Mac it has a little cog symbol in the top right corner. Click it.
If there's a sign out option then use that, but in my case there wasn't so I selected Preferences, then on the popup that opened selected 'Account' and then the 'Sign out of Creative Cloud' option.
After signing out I quit the Creative Cloud desktop app.

After that I launched FB 4.7 which now started properly. I wasn't prompted for a serial number at that point so I waited a minute, quit FB 4.7 and relaunched it. Now I saw the serial number prompt, entered my serial number and was all set.

Hope this helps someone.