Thats not a problem with the skinpack, its a bug with the mod, please contact for this the UnitedRacingDesign Team.
It's a bug with the current build. The virtual rides alternative way to add cars still works, if the skinpack is available in that form.
Ok i´ve found it here @ united simracing.net I´ve put the file ( http://unitedsimracing.net/uploads/downloads/projekte/sr-rf_dtm_-_v1.81.rfcmp) in the package folder, installed it trought the launcher but i don´t see any skin choosing the car . Any idea?