Angle Tool is only compatible with Blender 2.8.
There is a legacy version of the tool available for 2.79 called Corner Extrude, which you can find right here in the downloads.
The tool will give an error with Auto Merge enabled in Edit Mode. This will be fixed in the future.
Angle Tool is a simple and robust modeling tool for quickly creating corner geometry from any cross-section selection in Edit Mode. After selecting part of your mesh in Edit Mode and specifying an Active Edge, you can now use the Angle Tool command to turn your geometry around that edge and extend it to create a geometric corner. It's really simple but kind of hard to explain, so please refer to the following visuals :)
Angle Tool allows you to create this type of geometry on the fly and lets you avoid the extra steps that would otherwise be required to achieve similar results.
Extra features include the Curve Mode.