You should first quit out of DW and go delete the cache file, here are the steps for this:
common_installation_issues.pdf
Once you do this open up DW and go to Site > WebAssist Site Tools. This will open the site tools panel. Once it is on the screen dock it with the other open panels that you have. After this you should try adding in a menu to a page. Let us know if you have any further trouble with this.