Cannot install Web Assist extensions
the install process i use is:
Click the download link, if you have the option to open the file or save it, make sure to select the option to save, (in some cases like safari, it may be set to automatically open known files, this should be disabled)
after the extension file is downloaded, open the extension manager, and click the install button in the Top Right, then browse to the extension file that you saved to start the install.