If the Delta of 2 Documents option is not showing up in the Actions menu of the Notes client at all, then this maybe because the tool is not fully installed or has not initialised with Notes. Please check your Notes.ini for the line that starts:
It should have ntdelta as one of the parameters in the line.
If the entry is there then please restart Notes so the ini can be re-initialised.
If the line or ntdelta parameter are missing, then please add them.
If you are adding the whole line at the bottom please remember to hit Enter afterwards so there is a final blank line.
If the option does not appear and you have confirmed the AddinMenus line is correct then check you do not have duplicate notes.ini files on your machine and reinstall Delta.