First, make sure you've extracted the file correctly (you should be trying to open an application file). Also make sure you have updated your .net framework to at least 3.5.

Its some programming framework that is avaliable for free from Microsoft. You probably have older versions on your PC without you even knowing it. The one you're probably missing is 3.5.

That said, double check what kind of file you are trying to open. If it's an application ignore this. If its a .rar then you still need to extract the file (its like a zip file), I'd recommend using 7zip if you don't have a program for it already.