I notice that when all the tubes are on the internal flash really struggles to reach to the bottom of the subject, casting massive shadows. I'm still waiting on my external flash to arrive.. would an external flash work well on its own? Or is a shoe cord needed aswell?
The photos posted above were taken with my 580EX attached on top of the camera. For macro a ring flash would help get rid of all the shadows or I guess you can use multiple light sources.
Flash mounted on the camera works well enough, I would consider getting a shoe cord if it wasn't so expensive.
Edit: There was also a lightsphere imitation diffuser attached to the flash for the photos above. Definitely think about investing in a diffuser, a sto-fen or a lightsphere (cheap imitations on the bay works just as well).