Look at the documentation of EMGU you have: it should tell you the target version
of .NET it's using. Find out what .NET version do you need and install it. Do you use Visual Studio? What version? If you have VS 2005, you can develop only the applications targeted to .NET v.2.0, VS 2008 covers targets of v.2.0 to v.3.5 (and target version
becomes a project option found in the "Application" tab of the project's properties), VS 2010 allows you to target v.2.0 to v.4.0, and so on. Of find the older version of EMGU, as Wes Aday advised.
For advanced information on versioning, please see:
In particular, you will be able to learn about compatibility aspects which occur if you have assemblies targeted to different version of the .NET Framework. This is all your problem is about.