If you download a zip file from internet and unzip it to "My Documents" e.g. there are some security considerations which prevents .NET to load these assemblies.
for this can be found
You can identify this issue if you have log entries like this in the main log file:
Exception: An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this
load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See http://go.microsoft.com/fwlink/?LinkId=155569 for more information.