I should have added this to my previous post. You can use older Nikon flashes and 3rd Party dedicated flashes for Nikon, but they will only work properly in the Automatic or Manual mode. They will not work in TTL mode on the D100. As mentioned, this is a shame and one that Nikon could have corrected. Contrast this with the Fuji S2 Pro that got the flash scenario right by allowing a wide variety of flashes to be used in the TTL mode. Include other Digital SLR's in this category also.
What you would have to do is to set the Sunpak to the Auto mode or manual mode. Refer to the manual for detailed instructions. Then set your camera to the aperture recommended. Again, check the flash manual for specific instructions and your D100 manual for instructions on using in Automatic or manual mode with flashes.
To use a flash the works with TTL, however, you will need to dish out the money for the SB-80DX or SB-50DX.