How to activate an external flash on a Nikon D3000?

I'm trying to use a PROMATIC FTD 2500 with The NIKON D3000, but i can't get the external flash to go off. Any information is good.

You have to program the camera to see the flash. Check your owner's manual for details.

It looks like that is an old, and maybe Minolta or otherwise not Nikon flash. I do not believe it will work at all for your Nikon D3000. You need to have a flash that has the right electronic connections as well as the right computer communications, i.e. One that is compatible with your camera. Consult your owners manual for what flash units are compatible.

I'm not familiar with both camera and flash but if the flash has TTL mode for the Nikon dSLR, use that. If none, here's what I do with generic flashes.

Set camera to manual mode and use a shutter speed of 1/60 (or whatever the maximum flash sync speed of the camera is). Set the flash to Auto or Manual. Focus on subject. Read the distance and check on the guide at the back of the flash (not necessary in Auto mode). Use recommended aperture size from the back of the flash (do it also in Auto mode). Fire!

This is a flash that uses modules to allow it to work with various camera systems. Even if it is a Nikon module, it is a different form of TTL from what your D3000 understands, and I know of no way to make it work, via TTL.

Now, if yours has the switch on the module that says "1/16, M, A, TTL", you may still be able to use the flash. You can't use the TTL position, but the others should work. They won't use the TTL pins, only the center X-sync contact.

A is Auto mode, meaning you set the flash and camera to the same ISO and aperture, shutter speed below max sync, and the flash will control it's own output.
M is manual, the flash will fire at full output every time.
1/16 is just that… 1/16th power. The flash fires at 1/16th every time, giving really fast recycle times, but very short range.

Try the non-TTL modes. If none of them work, then it is possible the module is broken, the flash itself is broken, or (most unlikely) your hot shoe it broken.