If the plugin activation window isn't already open, click on the "Activation Required" text, or the key icon on the bottom right side of the plugin window.  Next click the "I'm offline" button, and then click "Save to file" to save your machine ID.  

Then, on your online computer, go to Manage Devices, click "choose file," and upload that machine ID file that you got from the plugin on your offline computer.  

Click "activate."  Then click "Download license file" and you'll get a file that you can upload on your offline computer, by clicking on the "open license" button in the plugin window.