It should work if it is compiling i.e TreeNode comes from CMS.DocumentEngine namespace. You probably looking at the wrong document. Check node.DocumentCulture and node.DocumentName or node.DocumentID just to make sure that you looking at the right document.
I see that you are resolving macro so your node.DocumentMenuClass contains some macro {%somemacro%}. If somemacro fails - you get empty string.