You download the zip file, unzip it. Move the folder ("UnrealMultiSpot" in this case) into your "FPVFreerider_Recharged_Data" folder.
(In this case, you also need to rename what the creator named "custom1.txt" to "UnrealMultiSpot.txt", since the .txt file name and the folder name needs to be the same in order for Freerider to find it).
There are more info on custom levels in the user manual.