How to use a hammerhead flash unit with my Nikon D90?

I have some manual flash units and a bracket system from my old film camera. I would like to use the bracket system with my D90.Is it possible?

If you have the PC cord for it, a hotshoe to PC adapter can allow you to use your flash in manual mode.
BUT, it is not known if the trigger voltage is safe for digital cameras. (This was not a problem with mechanical film cameras.) If you can't confirm that your unit is safe, instead of a dumb adapter, you need a safe adapter like this:
http://www.bhphotovideo.com/...oe_to.html" class='ext_link'>http://www.bhphotovideo.com/...oe_to.html

Http://www.botzilla.com/photo/strobeVolts.html
According to the above site, it might be useable on your camera. Nikon DSLR cameras generally can handle 250 volt trigger voltage (see your manual, do a search in the pdf version of the manual for "volt"). If you want to use the bracket with your camera, you will need the appropriate connectors, probably PC. If your flash uses a PC connector, then you will need a hot-shoe to PC adapter which any photo store carries. You can also get a flash cord like this:
http://www.bhphotovideo.com/...d_for.html
This cord has a PC jack on it.

About trigger voltage, you need to measure the trigger voltage at the flash terminals. You need a high impedance meter, 10 megohms or higher. Measure the voltage. If it's 200 volts (takes the high impedance nature of high voltage trigger terminals in to consideration) or higher, don't use this flash with your camera unless you get an adapter like
http://www.bhphotovideo.com/c/product/245292-REG/Wein_W990560_Safe_Sync_Hot_Shoe_to.html