Unfortunately, Nikon recommends that you use their Nikon DX series of flashes. These are designed to work with digital cameras, whereas other flashes - whether Nikon or 3rd Party dedicated flashes won't. You may have to resort to either Automatic settings or manual as some have done with other than the Nikon DX units. I'm no technoweenie when it comes to flashes, but I do know that Nikon failed - in my opinion - to properly address the flash scenario with the D100. Contrast this with the Fuji S2 Pro that was designed to handle all types of Nikon and 3rd Party dedicated flashes and performs far superior with these units. Although I have Metz and Sunpak units - to include the Nikon SB-28, I had to go out and purchase the SB-80DX in order to work synergistically with the D100. I, luckily, also have the S2 Pro which allows me to use my Sunpak 5000 and my Metz 54 - plus my SB-28!!!
I'm sure you will generate alot of responses from other readers who have figured out ways of using their older flashes to work with the D100. I'd like to see some of those, because flash is my weak point.