When should my seedlings go under the light in my expando garden? Do I wait until there are leafs?

The seedlings require light as soon as they germinate, otherwise they will s-t-r-e-t-c-h looking for light. The light should not be too strong at first. If you are using a metal halide, simply keep them off to the side a bit (not directly under), keep the light at 3' or so above and diffuse the light with a piece of thin white plastic.
You can place them in your expando at the 2nd set of true leaves.

I have a 400W Metal Halide and I’m trying to germinate seeds. What distance should my lamp have from the sprouts?

When germinating under halide keep the bulb at the ceiling. You can diffuse the light by placing white plastic over the container, a common white garbage bag will do. The light is very intense (and you will be thankful later when your plants are bigger and growing rapidly!) so 4' is the absolute closest the bulb should be from the containers. The light can be lowered slowly as the plants grow till they are within a foot of each other. A 400 watt is good for your size area.

What is a good temperature for germinating seeds?

The temperature should be no higher than 74 degrees F. Too high of a temperature would fry the germinated seedlings. (in older plants it causes them to stretch, so it is an important factor throughout the growth cycle)

A germinated seed should have sprouted in one week.

My seeds are in Rockwool, I did water almost daily. Could the germinated seed have rotted?

Any problems in Rockwool can usually be countered quickly. Because you mentioned the bean seed had rotted, it seems to me that the seed could not breathe. The Rockwool must have an air space underneath it. Is it possible that the cube was sitting on a flat surface? If so, the bean surely would have rotted. A germinated seed should have sprouted in one week. The temperature should be no higher than 74 degrees F.

Homegrown Hydroponics Inc. - http://www.hydroponics.com