A diffuser will not help images that are "too dark". That is due to you not setting your camera up properly. Look at what ISO/Shutter Speed/Aperture you are using.
A diffuser is used for softening and bouncing light and reducing shadows.. It does this..
http://www.stofen.com/Info/HowItWorks.htm
The Gary Fong diffuser you linked to is pretty good, but to be honest a piece of tissue paper in front of a pop up flash also does a good job.
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