Thank you for your question.
What version of dwf viewer are you using? The latest version of dwf viewer - Autodesk Design Review 2013 is now available for download from our website. This product can let you add markups to a dwf/x file.
Any furhter question please let me know.
Herbert He
SW Engineer
PSEB-GE- ACRD PSEB
Autodesk, Inc.