How to set up an external flash on Nikon D3100?

I have a d3100 and found an external flash at my friends house. Its a Bower SFDS. I mounted it right to the hot shoe but I can't seem to figure it out. I know the flash is on but there's no test button. I just need to figure out how to actually make it work with my D3100.

Don't use the flash unless it was specifically made for Nikon DSLR cameras

the reason is that each flash has its own trigger voltage, meaning, to trip the flash, an x amount of electricity from the camera has to come. If it is too high it can basically fry your DSLR

Dr Iblis has a point but has gone a bit over the top. Most modern digital flashes have safe trigger voltages - it's the older ones designed to be used with film cameras that are the digital camera killers - they have trigger voltages of maybe tens to hundreds of volts & most digital cameras don't like trigger voltages much above 9V.

I have no info on this particular flash - it's not listed here;
http://www.botzilla.com/photo/strobeVolts.html

however, if you have a multimeter you could test it - instructions here;
http://www.botzilla.com/photo/g1strobe.html

If you can't do this, it's just too risky taking a chance. A cheap manual (safe, modern) flash will do just as well - a Yongnuo 460-II costs around £40 and will work fine. Get a set of RF602 triggers (£25) and you can get a flash OFF camera (much better!).