You'll find Tap Tempo in the Quick Tempo Entry page. You can also have the main tempo display work as tap tempo.
To access the Quick Tempo Entry page simply tap on the tempo. You'll find the Tap Tempo button in the top right of that page.
To allow Tap Tempo on the main Metronome Page go to Preferences in the PolyNome Menu and turn on "Tap Tempo On Main Page".
When this is turned on, tapping the main tempo display will function as Tap Tempo. You'll then have to tap and hold to access the Quick Tempo Entry page.