The most reliable way to enable ALT+TAB I have found is to edit the wfclient.ini file. This file is available whenever you install the Citrix Receiver client.
It can be found in the following locations:
Linux and OSX
In that file you want to find the [WFClient] and add the following to the end of the file.
Restart your ICA or Remote Access session and enjoy. This should work whenever the session has focus regardless of if it is full screen or not. If you only want it to pass through when its full screen, then change Remote to FullScreenOnly.