Updated Solution
I was able to get Pex to run in VS 2010 with .Net Framework 4.5 installed. Maybe others have explained that this is the solution but my issue (being new to Moles and Pex) is that many of the explanations were vague. Thanks to the author of these posts because they got me headed in the right direction:
http://feherzsolt.wordpress.com/2013/02/13/moles-and-net-4-5/
http://www.daedtech.com/using-moles-with-the-system-assembly
So the solution is to create a new .moles file (System.moles) in the test project at the same level that Pex created the yourAssemblyUnderTest.moles file and add the following to the file.
<Moles xmlns="http://schemas.microsoft.com/moles/2010/">
<Assembly Name="System" ReflectionOnly="true" />
</Moles>
You may have to clean (or manually delete the obj folder contents) and rebuild but this solution worked for me. I also noticed that the first build would report build errors, but the errors would go away on subsequent builds.
Alternative Solution
I know this isn't THE solution but it is a (poor) workaround that allows Pex to run...plus this was too long for a comment. If anyone knows how to get PEX to run on a machine with the 4.5 .Net Framework PLEASE respond.
This was my scenario. I have a machine with the .Net Framework 4.5 and I have Visual Studio 2010 and 2012 installed. I installed Pex (en_visual_studio_2010_pex_0.94.51023.0_power_tools_x64_598803.exe) and could run Pex in a VS 2010 project that was targeting .Net Framework 4.0. I could run Pex and it identified issues...but once I promoted issues to Unit Tests the unit test project wouldn't compile because of the following build error
The type or namespace name 'IReadOnlyList' does not exist in the namespace 'System.Collections.Generic' (are you missing an assembly reference?)
After much searching, and reading that Pex or Moles was replacing 4.5 .Net Framework assemblies with 4.0 versions I decided to uninstall 4.5, install 4.0, and give it a try. Running on 4.0 works just fine and I didn't need to define Mole type filters to get it to work. Then I tried tried to upgrade to the 4.5 Framework again...but sadly I end up right back where I started with the IReadOnlyList namespace issue.