Thief

First, you need to select the "Thief" sheet via the "Sheet" selector in the upper right of the character sheet titlebar:

Then, you need to choose "show backstab" for the sheet using the "Config" button in the upper right of the character sheet titlebar:

To enable backstab for a weapon, edit the weapon and check the "backstab" checkbox:

This should enable a new icon on the equipment tab for backstab.

You can roll Thief skills from the Thief tab.