The Citrix Desktop Viewer toolbar if officially supported with XenDesktop only, but also works with XenApp. The Desktop Viewer is enabled on the each Web Interface/Web Services site in WebInterface.conf by adding the line ShowDesktopViewer=On
The Desktop Viewer can also be enabled and disabled in default.ica on each Web Interface site where ConnectionBar=0 disables and ConnectionBar=1 enables the Desktop Viewer. This setting is found in the [Application] section. However, when ConnectionBar=1 is set this breaks seamless applications - they are seamless no more and have frame with the Desktop Viewer showing which is probably NOT what you are after! I recommend using for former method of setting ShowDesktopViewer=On only.
Note that once the Desktop Viewer is enabled the display settings from the farm and Web Interface are ignored – the idea being that Desktop Viewer gives the user control of how to size and place their desktop session. Enabling the Desktop Viewer on a desktop appliance site where a published desktop automatically opens full screen will break this functionality; the desktop session will open in a window instead.