Please use the Manage Devices page to activate new plugins onto an existing machine, instead of the Manage Registered Plugins page.  It's faster if you're buying more than one plugin at a time, and it ensures that all your plugins are authorized consistently.

Go to the Manage Devices page, and click on the dropdown menu in the "Activate Existing Device" section.  Select the name of the computer or USB stick you want to activate, and then click the "Activate" button.  Our system will automatically license all the plugins in your account that aren't already licensed to that machine.  

Then click the "Download license file" button at the bottom of the page, and you'll get a file named "pa.license." Save it to your flash drive and go back to your offline computer.  Start up ProTools (or any DAW) and open the plugin window and click on the "Activation required" text or the key icon on the lower right corner of the plugin window.  The activation prompt will appear.  If necessary, select the appropriate device under the "Select the device you would like to activate" text.  Next click the "I'm offline" button, then click on the "open license" button, and select that pa.license file. Then click "activate."