Make sure to download the MXP File to your computer beofre installing.
When you click on the download link, you may have the option to Open it or Save, select the option to save it.
once the download completes, open the extension manager. In the extension manager, click the install link, then browse to the location where the MXP file was saved to launch the installation.
after completing the installation in the extension manager, open DW. This should launch the WebAssist Installer to complete the installation. If the WebAssist installer does not launch automatically, you may need to open a php page to trigger it.
After the WebAssist installer completes, make sure to restart Dreamweaver.