Could not load file or assembly The system cannot find the file specified

My PowerShell ISE wasn't running as an Administrator. This seemed to be the problem for me.

Right click on the project you created the dll and the new referencing project, then select properties. Under Application, check the target framework and verify that both have the same framework, some dll projects tend to select 'client profile' version of the framework by default, which tend to give the error you are having now..

Let me know if this is not the issue..

As pointed out in the comments, SysInternals' Process Monitor is a valuable tool to diagnose DLL resolution problems. Tells you when a 3rd party DLL has a dependency you don't know about, also tells you when Windows is looking in the wrong corner of your hard drive for the file or finds the wrong one.

Loader snaps is the built-in diagnostic tool for Windows. But Process Monitor is far more convenient.

It does generate rather a lot of information, start from the bottom of the trace or enable tracing at just the right time. You often need to use its filtering tools to turn the firehose in a relevant trickle. Worth the hour of your time to figure it out, this tool belongs on any programmer's black belt.