The tray icon should be at the top right end of the taskbar, next to the date, user name and spotlight.
If the tray is not on screen you can make NI mate start with the control interface shown. First download this preferences file. Then open the terminal and:
./Delicode_NI_mate -preferences /Users/youruser/Download/nimate_notray.prefs
Change the path to where you downloaded the preferences file.