what version of dreamweaver?
DW CC? or CC 2014?
Make sure you have the latest updates for the extension manager installed.
When you open the extension manager, do you see the MySQLi server behaviors listed?
After installing the MySQLi extension, when you open DW, it should launch the WebAssist installer. if it does not launch the WebAssist installer automatically, open a php page to trigger it, make sure to restart DW after the WebAssist installer completes.