Nikon D3100 flash unit?

I have recently got a canon eos rebel ti 35mm and just now a Nikon D3100. Today i picked up a Promaster FTD-5600 Dedicated Flash at a used store for $15 and it seems to work fine with both. Being new with photography is there any issue with using this with a digital camera etc. Also any tips for using the flash unit would be much appreciated.

I was warned that some flash units have a higher voltage and could "fry" my Canon DSLR.

I have no idea about that particular flash, but I'm sure you wouldn't want to risk damage to your camera.
I'd very VERY carefully research it first.

Not smart, using a strange flash without measuring it first. Any damage will have been done by now.

Err.use it, see if you can figure out how to underexposure by 1.5 stops, let the flash fill it in and do so somewhat indirectly.

First… Congratulations on having the budget to use both Canon and Nikon cameras and build an extensive collection of lenses for each. Most people choose a brand and stick with it.

Second, from what I have read on the FTD-5600… You need to buy lower units for the flash for it to be fully compatible with your cameras. You would need the Canon AF CN and the Nikon AF NK modules… Since you have evidently put it on your cameras, it has a module on it, but you need to determine which one it is. See page 2 of the manual. Here is the flash manual if you don't already have it… Http://www.promaster.com/support/books/Instructionmanuals/FTD5600.pdf

Http://www.botzilla.com/photo/strobeVolts.html is usually the place to check for compatibility. Your flash isn't listed, but other model numbers around it appear to be safe for Canon cameras (limited to 5 or 6 volt trigger voltage). Find someone with a voltmeter and measure the trigger voltage on the flash to be safe. The above link has instructions on how to measure the trigger voltage.

On the D3100, according to the manual (page 172), it is rated for 250 volts, so you'll probably be OK with the Nikon.