How to make a stargate work ??

You might want to be more specific on what you need done. Is it the buttons that trigger another object? What is DHD? If you have a stargate object, you could send messages from the buttons to the stargate, but this is still too vague to help you.

Stargate : A stargate is a fictional device that allows one to travel through a wormhole to distant locations. By the distant locations, it is to another planet and even another galaxy. There are 9 chevrons. These chevrons act as a 'dialling address' a navigation system to the location the stargate a 39 symbols on it. photo of a stargate http://upload.wikimedia.org/wikipedia/en/7/7a/SG1stargate.jpg

DHD (Dial-Home Device) : A DHD has the same 39 symbols the stargate haves, it's is like a phone once you have pressed the right 7 symbols on the dhd the stargate will get connection to a adress. each time and a symbol is press and chevrons on the stargate will light up red or blue.

Photo of a DHD : http://upload.wikimedia.org/wikipedia/en/d/dd/Dhd.jpg

The stargate and dhd works like in this video http://www.youtube.com/watch?v=6Wdjc1LrLPg

there is so many different ways to do something like this but primarily you want to have gui or objects and then test for a hit on each, and when each button have been pressed load a level, so you could use a simple counter which adds one when a button is pressed and if counter == to 5 play a little cutscene and then loadlevel

A few things still need to be cleared. If you are using a 3D model of the DHD, or just 2D. Either way, you will need a system to press the buttons and another to check for the current pressed buttons. The video indicates a sequence is necessary, so you'll need to check for this too. Also, you will need to figure out how you want the buttons to trigger the lights on the stargate.