Depending on the FORTE version you're working with, there are different options for entering guitar tabs.
Select the tab "Text / Chords" from the input palette and click on the chord symbol.
Bring the mouse to the measure you'd like to enter the chord in. The ruler will help you finding the right position. Once you have the right position, click.
A new window, "Insert Chord", opens.
Select the desired chord.
Under "Style", select "Chart", if you wish for a chart to be displayed on your sheet music.
Confirm by clicking on "Insert".