The new version of Mono (1.1.9) changes the error message format around a little. This is the code I put in my .emacs file to get Emacs to understand that error message:
(require 'compile)
(add-to-list 'compilation-error-regexp-alist
'("\\(.+\\)(\\([0-9]+\\),\\([0-9]+\\)): \\(error\\|warning\\) CS[0-9]+:" 1 2 3))
