Noobie : Help wanted : Activation

Hi
Wonder if anyone can help me with activation of Unity. I am trying to buy the Android licence but it is greyed out and the box does not accept my Free Serial Number which I got from Help/About Unity/Serial Number. My Unity version says Developer Preview
Version 3.5.0b6.

I have been trying for 12 days and contacted support but no go. Maybe I am not doing the right procedures, I don’t know where to find out.

On their website there is a page with

//////////////////////////////////

Activate Unity
Thank you for choosing Unity.

If you have a valid serial number you can use that to activate your product, if not you can get a free activation:

Free activation

Serial number
Licence request file :
[Browse]
[Activate Unity]

//////////////////////////////////////////////

I don’t know what any of these words mean or where I browse to, or sequence of
Events, or what is a VALID Sn.

Can anyone help ?

Thanks

Lennie

Do you have a support case number?