My photos are coming out dark on my Nikon D5100...should I be using an additional flash? An extra one attached to the camera? Sometimes they look dark on the live view and come out okay after the picture is taken when I look back at them.

The build in flash, will be useful within several meters (15 to 20 feet) if it is really dark.
Your camera should be capable of taking pictures in poor light conditions. Try to shoot in automatic mode. The mode dail to AUTO. (green camera symbol). Then view the picture with info. There you can see the ISO, Aperture and shutter time. From there you can experiment for darker or lighter pictures. For this you can choose M, A,S or P.
In each of these modes, you can use the following steps.
On top of the camera, close to the shutter release button you see a little knob with a +/- sign. If you press that button, you should see a 0.
If you see a - figure, this means the picture you shoot all be darker than normal. If you see a + sign with a figure it means the picture will be brighter. While pressing that button and turning the command dail, you can choose -3 to +3 EV. That is much darker and much brighter.
If you shoot many pictures in bad light conditions, you could buy an external flash. A Nikon flash will work very good and automatic, with your camera, because camera and flash will communicate and help each other to make better pictures. Cheaper external flashes can work too, but most of the time you must change settings manual on the flash. Never use old flashes, that were build for analog cameras. They sometimes switch with a voltage higher than your camera can handle. The camera can be damaged by such a flash.