MAC - Engine - build failure - mscorlib - Mono error

Dear Community, I am unable to build the UE4 Editor on my Mac.
When building the GitHub Release - like told in the description - using the GenerateProjectFiles.command file I got the following error:
(Mono was installed using the seperate installer)

Setting up Unreal Engine 4 project files...
Setting up Mono
The assembly mscorlib.dll was not found or could not be loaded.
It should have been installed in the `/Applications/UnrealEngine/Engine/Binaries/ThirdParty/Mono/Mac/lib/mono/2.0/mscorlib.dll' directory.
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When duplicating and renaming the 4.5 folder containing the needed mscorlib.dll file to 2.0 - like in the above error wnated, another error pops up.

Setting up Unreal Engine 4 project files...
Setting up Mono
Cannot open assembly '/Applications/UnrealEngine/Engine/Build/BatchFiles/Mac/../../../Binaries/ThirdParty/Mono/Mac/lib/mono/4.5/xbuild.exe': No such file or directory.
Cannot open assembly '../../../Binaries/DotNET/UnrealBuildTool.exe': No such file or directory.
logout

Am I missing something that needs to be installed which was not written somewhere where I could have found it or am I just simply missing the point here?

XCODE is up to date. Version 6.2 (6C131e)
For the sake of completeness my System Specs:

Mac Pro (Late 2013)
3,5 GHz 6-Core Intel Xeon E5
32 GB 1866 MHz DDR3 ECC
AMD FirePro D700 6144 MB

Thank you very much for your help/insight/solutions!

Looks like Setup.command was either not run at all before you started GenerateProjectFiles.command, or did not download all necessary files (if that’s the case, hopefully it shows some errors). If it’s neither of those, though, let me know, I’ll give a list of things to try.

Michael,
thanks a lot for your reply.
It seems that may be the case, I started it and it is now receiving a lot of updates - will take some time, but I guess that this was the problem.

Thank you very much for your fast and helpful response!