The red exclamation mark on your Xmarks icon indicates a sync error. Do the following:
Make sure you're running the latest version of Xmarks for your browser. The latest versions of Xmarks for each browser is available on the Xmarks Download Page. -- Your version number is noted in your Xmarks Settings (Xmarks icon > Xmarks Settings) under the Status tab. (Example: v4.0.3)
Follow these steps to disable all other bookmark sync extensions running in addition to Xmarks.
Perform a repair followed by a sync.
If your repair is unsuccessful, please attach your full log file (as a .txt file or .zip file) following the error message to a support ticket for further review.