This is a known issue with building .NET 4.0 libraries and using powershell.
This issue is more of an FYI than a problem. I have done the following workaround but it is something you should consider as the other prowl.net library is currently supported for Powershell without recompiling the assembly or editing the config files.
Details
http://connect.microsoft.com/PowerShell/feedback/details/525435/net-4-0-assemblies-and-powershell-v2#