If you're using the desktop version you can also use rulers to draw straight lines with. You can find rulers in the stencils panel. These can be placed on the canvas, moved, scaled and rotated and you can paint along the edges of them with any tool. See the rulers section in the manual ( Help -> ArtRage Help or Help -> Manual depending on your version ) for more information.
If you have any problems, let us know and we'll be happy to help. If you have any further questions, let us know which version of ArtRage you're using as this will help us to give you specific information.
Resident Bug-Hunter and Support Guy