Full disclosure: I work for Telerik.
Please follow meilke's comment.
Nevertheless there are two options you can try:
As the license is developer based, it's important what email you use for downloading the licensed version. You should use the email that is assigned as licensed developer when downloading JustMock. I guess you've received similar answer from the sales support team.
The second option can be if you have used the JustMock trial version and you have included a trial Telerik.JustMock.dll somewhere in your source-control system. Hence despite installing the licensed version, your tests run with the trial.
Hope this helps.