It looks like the WA Installer has not run to complete the installation.
In Dreamweaver, reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
Then Close Dreamweaver and Delete the Dreamweaver Cache file from the users configuration folder:
Open the extension manager, uninstall the MySQLi extension and re install it.
Open DW Again, and make sure the WebAssist installer runs to complete the installation, if it does not start automatically open a php page to trigger it.
once it completes, make sure to restart DW.