Much, much activity at the Technical Support desk about this question.
Here is the process:
1. Log into your Arturia account, go in the Licenses section, click on the "Get FREE UPDATE" button aside your synthesizer registration.
2. You'll be prompted to a confirmation page, read it attentively and click on "Submit".
3. Then you will be shown the direct link to update installer, and the activation code needed to download a license in the eLicenser Control Center.
4. Install the latest version of your favorite synthesizer. Do not launch it, as you did not authorize it yet! (it will inevitably crash)
5. Once the synthesizer is installed, launch the eLicenser Control Center from the Applications Folder, click on "Enter activation code" button on the interface, enter the code, click Next, download the license on the eLicenser (being Soft or USB).
Hope this helps, if not, contact the Technical Support desk directly from your Arturia account.