Make sure your flash drive is in your computer. Start up ProTools (or any DAW) and if the activation window doesn't come up automatically, go ahead and add one of your PA plugins to a track and click on the key icon on the lower right corner of the plugin window. Under the text "Select the device you would like to activate," there is a pulldown menu.
Click it and select your USB drive. Make sure the "Activate all my licenses" checkbox is checked. Next, click the "I'm offline" button and "Save to file" to save your machine ID to the flash drive. Then, on your online computer, go to Devices > 'Manually activate new offline device', click 'Select' and upload the Machine ID file you got from the plugin on your offline computer.
Click "activate." Then click "Download license file," and you'll get a file named "pa.license." Save it to your flash drive and go back to your offline computer. Click on the "open license" button in the plugin window, and select that pa.license file. Then click "activate." Now your USB drive is authorized and can be used on any computer.