Hi,
To be able to run the DLC levels you need the latest version of FPV Freerider Recharged.
When you purchased the sim you got an email with a link to your personal download page, you can download the latest version from there.
In case you have lost that email you can request a new one here:
https://itch.io/support
(Under "recover purchases")
(If you are unable to load the custom levels, make sure you have the latest version of Recharged installed, released february 2018. At the time of this writing, the downloadable file for the latest version for Windows is called "FPVFreerider_Recharged - 2018-02-19.zip")
Hope that helps!