Are you trying to use the Online version? The code is only to install the Desktop program. After you install the program then when it starts it will ask for the code.
I have Windows and downloaded the program. I logged into my account and went to Downloads. I first clicked on Copy Code then on the download. After it downloaded I clicked on the exe file. Got a little worried when it didn't ask to Activate when installing. But when it was done I clicked on Launch Now and the first thing the Activate box popped up and I did Ctrl+V to paste the code and it worked!
