First I get errors that the Antlr namespace can't be found, which somehow got fixed by changinig the framework version to 4.0 from 2.0.

Now I have these errors:

Error 2 'Func' is an ambiguous reference between 'System.Func' and 'Jint.Delegates.Func' C:\Users\pc\Desktop\Jint - Sources\Jint\Native\ClrImplDefinition.cs 24 34 Jint

I started changing every occurence of Func<> to System.Func<> but more and more of those kept popping up and before I change hundreds of lines of code, I thought I'd ask the experts. Can you help me? 

The project is compiled against C# 2.0 and CLR 2.0. Which explains. There are advantages and disadvantages to that.

The sensible thing to do at this point is to have the Jint library #ifdef'ed so that it can compile under the CLR 4.0.