I think Jan is correct; the lighting looks too flat. There are two problems I see - first is direction, and second is that the light is too soft.
Your flash looks like it has an optical slave - you should be able to use it off-camera by using the D5100’s pop-up flash to trigger it (you’ll lose iTTL though, and will need to be shooting in full manual - flash and camera.) Just put your pop up flash at minimum power and add a card between the it and the subject, so that it doesn’t provide any light. This should allow you to add some direction to the light.
Lock your shutter speed at 1/200s; start with the aperture at f/8, and your flash at 1/16 power, close to the subject. Take a test shot and chimp - if the subject is underexposed, open your aperture or increase the flash power. If the subject is overexposed, either close the aperture, drop the flash power, or move the flash farther away. Once you get a good “feel” for your flash power, you could work on using ambient light to control the background exposure.
Second is I think the diffuser is too large for macro work. Spreading the light over too large an area makes it directionless. (It’s almost like taking a photo on an overcast day - when it’s clouded over there are no shadows, and thus no direction.) Since you’re doing macro, you could probably do away with the diffuser altogether, and just use bare flash. You might also want to try using a gridspot instead of a diffuser.