I solved it... here's the REAL problem.
Your computer is blocking activex controls.
The first solution is to go into Internet explorer>Tools>internet options or control panel > internet options.
Under the security tab, choose internet zone. Click custom level, and enable the options pertaining to ActiveX ontrols ans java scripting. Then you SHOULD be able to view the controls that were blocked out in the blank activation screen.
This didn't work for me, but it works for others, so I hear.
The second solution for me was to enter the C:\Program Files\Common Files\Autodesk Shared\AdLM\R1\en-US\Webdepot folder, which is where the registry HTMLs are stored. These are the pages that the blank screen shows on.
Open the RTBeginReg.html in a text editor like notepad. Add the following line of code below the <html> line.
<!-- saved from url=(0016)http://localhost -->
This marks the page as SAFE. Save and you should be able to enter your code and stuff.
Someone above wrote this, I spent hours trying the other ways, this is the only way that works for Windows 7
Here goes... You right click on the RTBeginReg.html icon when you find it in that folder and tick all the permissions, this gives the administrator full control over saving the file in Notepad meaning it can easily be saved and hey presto it will activate.
Hope it solves peoples issues, thanks