The Tablature Input type is purposed for adding notes directly to a tab. It lets you to edit fret numbers for each string via Frets numpad.
Select Tablature in the Input types popover. In the Actions Tools panel tap the String button (e.g. the 1st one) to open Frets numpad.
Using Number buttons enter the fret number for selected string. The numpad allows you to enter frets from 0 up to 32, like in a calculator. Empty fret deletes note at the current string. To close Frets numpad use Done or tap again on active String button.