Licenses for Load Tester are all online, i.e. no downloading of an actual file.  
1. Create Portal Account
When you purchase Load Tester you will have received an email with a link to create an account.  If you already have an account please notify support on this ticket so you can be added to the users for your company.
2. Configure Load Tester for your Portal Login
Use Help->Licenses to go to the portal account configuration dialog.  (See attached screenshot.)
Click on the Portal Account tab in the left-hand column and enter your username and password, being sure to check them using Verify Button.

3. Activate Licenses
If the license is currently being used by someone else, they will need to deactivate the licenses first.
Click on Licenses tab in the left-hand column and click on the Check for Updates Button.   You can then select and activate your desired license by clicking on the Activate Button.

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4. Multiple Licenses
A single license is meant to be used by a single person.  While you can transfer licenses between computers, if you want two people to use Load Tester at the same time two licenses are required.