In most cases, you will not need to install separate USB drivers for the plugin to detect your device. The required USB drivers are installed with the plugin.
However, a few Garmin devices do require separate USB drivers for the plugin to work. And, if the plugin is installed in Internet Explorer using ActiveX, or as a Firefox plugin, the USB drivers are not included. If the test page indicates that the plugin is successfully installed, but it is not detecting your device, try downloading and installing the latest <a href="http://www8.garmin.com/support/agree.jsp?id=591">USB drivers</a>.