You can use sample code bellow:
CMS.TreeEngine.TreeNode node = treeNodeObj as CMS.TreeEngine.TreeNode;
int documentCheckedOutVersionHistoryID = node.DocumentCheckedOutVersionHistoryID;
CMS.WorkflowEngine.VersionHistoryInfo whi = CMS.WorkflowEngine.VersionHistoryInfoProvider.GetVersionHistoryInfo(documentCheckedOutVersionHistoryID);
string comment = whi.VersionComment;