Gah.. as mentioned yeah, that is the main. Now many boxes have the main in the same box as all the other breakers, but as you can see this is not the case. Do NOT "tap into" this, if you did you'd have a circuit with the potential to draw 100 amps, except the wiring couldn't handle that much, so it'd be bad news safety wise.
You want to go to the other box, regardless what your "electrician"/"contractor" said... (FYI I don't trust titles too much as it seems that anyone who goes to home depot enough times and owns a truck is a de facto contractor).
All that being said, if you don't know what you're doing, find a real electrician, and make sure that summabiznitch is licensed.