Windows 8 Enterprise defaults to using a corporate Volume License activation on a business network. This is supposed to provide a simplified end-user activation experience focused on using the KMS server, however i fyou want to use a MAK key then you are in trouble. This is because Windows 8 Enterprise PC cannot find the appropriate DNS records to determine how to automatically activate.
So the solution is to enter the MAK key manually from a command prompt:
Hit the Windows+R hotkeys to open the Run box and run the following command:
slmgr -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
You can then either wait for Windows to sort itself out - or click on the activate in settings and all should come right.
For MS's explanation see here