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Please see the official X‑Plane Support page for help.
There aren't any NDB icons on the Mobile app maps. So when you tap on a VOR (which are modeled), the option is to "tune Nav 1" or Nav 2. This changes the setting and deflection of the CDI, not the ADF (the instrument with a single yellow line). Perhaps the video you watched was based on the full desktop sim, which still models some NDBs.
More information on the differences and the basics of instrument navigation is discussed in the desktop manual.
This site is no longer being actively maintained.