Another good method for generating your AST is to use gplex and gppg - they are equivalent to gnu's lex and yacc respectively.They are written in C#, and MS uses the code in Babel (http://msdn.microsoft.com/en-us/library/bb165037%28VS.80%29.aspx)
gplex => http://gplex.codeplex.com/
gppg => http://gppg.codeplex.com/
To get my language started I used this:
http://www.iunknown.com/2007/11/lolcode-on-dlr.html - the source is here: http://www.iunknown.com/files/LolCode.zip
This doesn't work with the current DLR - mass hacking, slashing, burning and looting is required.
It'll get you started on how to write a grammar and wire it up to the DLR, then compile and invoke.
You'll also have to learn the LOL language in the process.
Someday someone will write an article on how to wire this all together, but until then it can be a bit confusing for someone that hasn't used lex/yac and/or DLR.
good luck, and don't even think about doing any of this without reading the documentation.