EXtension tubes and teleconverters (aka extenders, aka converters) are two different pieces of equipment that can increase your magnification. Tubes are just that- empty tubes that increase the distance between the rear element of the lens and the sensor, thereby decreasing your minimum focus distance. With the 100mm macro lens and no tubes your minimum focus distance is 5.9". With extension tubes you can greatly decrease that distance, thereby increasing magnification (the closer the lens is to the subject the greater the magnification). Teleconverters increase the focal length by their magnifcation rating. They generally come in two choices: 1.4x and 2x. Add a 1.4x extender to your macro lens and you now have a 140mm macro lens. 2x gets you a 200mm macro lens. With the 1.4x your minimum aperture becomes f4 and with the 2x it'll be f5.6. With that much magnification though, you'll be using a tripod and tiny apertures so the light loss isn't the end of the world. Sharpness loss will be fairly significant with the 2x and not bad with the 1.4x. Here's the thing though- I can't recall if the converters work directly with the macro lens. You may need a small extension tube in between because the converters stick out a bit in front and may not fit inside the opening at the back of the macro lens. I can't check right now but can later if you need to know.
Forgot to mention-- extension tubes come in a set [made by Kenko, 3 different size tubes] or you can get the Canon 25mm tube, which costs about as much as the whole Kenko set. I opted for the set.
