For notating fretted stringed instruments you can use tablature (or tab for short). Stave'n'Tabs, as its name implies, supports that type of musical notation and provides you with convenient tools to modify tablatures.
To edit tablature first select the Tablature input type in the toolbar or press . So the cursor starts indicating the current string for which you can enter fret numbers with digit keys on the computer keyboard. Navigation is done using mouse or keys.
Virtual fretboard in the Virtual Instruments board is another useful option for editing tabs. It can show labels for notes at their positions on the fretboard respectively to current tuning.