Create a References folder and copy all contents of SkaterXL\SkaterXL_Data\Managed\ to there

In order to have the built dlls in the game folder, create post-build scripts for each project you want to build. Follow the "PostBuildExample.bat" example. If you want to edit XLGraphicsUI's Unity project, you need to copy XLGraphicsUI.dll to XLGraphicsUI\Resources\XLGraphicsUI\Assets\DLL\

Post-build scripts should be like {ProjectName}\PostBuild.bat