If you do not have internet access on the machine you're trying to activate, you can obtain a license file using self-service, offline activation in your Xamarin store account, if you have purchased a license.
If you login to https://store.xamarin.com/account/Products/ and click on the activation code, you'll get a link to download the license file.
When you receive your license file, you should save it to the appropriate folder in your home directory:
Mac
~/Library/MonoTouch/License.v2
Windows
C:\ProgramData\MonoTouch\License\monotouch.licx
Once the license file has been saved to disk, Xamarin.iOS installation and activation should complete successfully.