Is the new computer you are installing on offline? If it has an internet connection, you don't need to bother with the key file. Just choose the second option for activation in the Activation Wizard, "Login to my FXhome account" - and enter your email and password, and the key file and serial stuff will all be handled for you automatically.
If the computer is offline, then I'd recommend re-downloading the key from your account, following the instructions in this article
. If that still doesn't work, please submit a support ticket, so we can make sure you get your keyfile, or work through whatever else needs to happen to get you up and running.