I have downloaded the csharp grammar project from here http://antlrcsharp.codeplex.com/releases/view/47523, and it runs fine.
But when I add ‘output=AST’ to the grammar, I get a TON of errors:

I am admittedly new to ANTLR, so I am not sure how to proceed at this point. I would have assumed this type would have existed in the antlr csharp runtime dll, but obviously not. Has anyone come across this problem before?
About a year ago I did a project with Antlr and ran into an issue where I got tons of errors. As odd as it sounds but by using an older runtime I got it to work. I think I used a dll that had been compiled for the 2.0 .Net Framework.
I do not remember exactly what the error was or what had actually caused it. All I remember is that it solved the issue. I have no idea if this will help in your situation but it’s worth a shot if you are stuck.