Retain 4.0 or higher
You will get an email with your validation keys, there generally be more then one, including one for server access and another for the module used for archiving from your email system.
Browse to the License Portal https://licenses.gwava.com/
Select the proper Product
Enter the Validation Key and click Next
Fill out the contact information and click Next
Download the .PEM file
If using Retain 4.1 or higher:
Browse to the Retain Server Web Console | Server Configuration | REST API
Browse to the M+ and Netmail Retain Migrator license PEM file
You should see it appear on the license list:
Create a REST Collector by providing a name and press the Add collector button.
Click the Export button and save the file to disk.
If using Retain 4.0-4.0.3.1:
Download a worker bootstrap file.