Please note that the following is not a supported configuration. At this time, Desktop Viewer is only officially supported and tested with XenDesktop. However, if you would like to use Desktop Viewer in your XenApp environments, please continue reading.
The information below is based upon the use of the Citrix Receiver 3.3 with legacy PNa and XenApp 6.5
Navigate to C:\inetpub\wwwroot\Citrix\\conf folder and open the WebInterface.conf in a Notepad file. there should be a line which will be commented like below
Uncomment the line by removing the hash symbol towards the beginning of the line and you are done.
In some cases, there will not be a line for DesktopViewer at all in the WebInterface.conf file , if that’s the case, please continue to add the line above ( marked in yellow) and save the file.
If the above setting doesn’t work for Web Interface 5.4, try the Citrix KB here http://support.citrix.com/article/CTX122544