I was installing latest MonoX on our designer's machine and although it run fine, when I tried to upgrade our current database to it and run it I got:

Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'

I searched for that and found:
http://stackoverflow.com/questions/13748055/could-not-load-type-system-runtime-compilerservices-extensionattribute-from-as
which says that the issue is cause because an attribute has changed assembly or namespace or something like that from .NET 4.0 to .NET 4.5. This shouldn't be a problem because of some type mapping being set in the system, but if you've used ILMerge with some wrong parameters then it can cause this error when machine doesn't have .NET 4.5 but only 4.0 and tries to load the merged assembly

from that the following are mentioned:
http://marcgravell.blogspot.co.nz/2012/09/iterator-blocks-missing-methods-and-net.html
https://connect.microsoft.com/VisualStudio/feedback/details/726702/moving-system-runtime-compilerservices-extensionattribute-to-mscorlib-breaks-structuremap-scans-of-namespaces-containing-extensionmethods-and-possibly-other-ioc-scenarios
http://www.mattwrock.com/post/2012/02/29/What-you-should-know-about-running-ILMerge-on-Net-45-Beta-assemblies-targeting-Net-40.aspx

so I installed .NET 4.5 from:
http://www.microsoft.com/visualstudio/eng/downloads#d-net-45
rebooted the Win7 machine as prompted

and that issue seems to be fixed now, but I'm getting other error (please see separate post)