MendelMax 1.5: No power to Arduino when RAMPS is connected

Hi! I just finished the assembly of a MendelMax 1.5. Here are my troubles... I plug in the power supply (I used a regular cable that is used for a PC with the black [live], white [neutral] and green [ground] wires), and the power supply light turns on. If I have a fan plugged into the RAMPS board, it immediately comes on, so I know I have power going to it. Now, the Arduino board that RAMPS is plugged into does not appear to receive any power. I tried plugging it into my computer's USB port but nothing happens. Now, the minute I disconnect RAMPS from the Arduino board, the computer will detect the Arduino board (the drivers for it are already installed) and assign it a COM port. I can even tell Repetier-Host to Connect using that COM and it will do so. But obviously I can't issue any commands to the printer since RAMPS isn't connected. I'm using everything that came with the kit. I have a 12V / 29A power supply. All the wiring is correct. Positive/negative to positive/negative, etc. I know the Arduino board has an AC plug but I read that you shouldn't attach anything to it as it receives its power from the USB. Also, just in case you need to know, both Diode 1 and 2 are soldered onto RAMPS.
I'm not sure if it's normal or not, but when I plug in the power supply, the Arduino board gets really hot in a certain area (the little chip right beside the power plug.
I wanted to try plugging in another power source to the Arduino board (like a 9V battery), but I'm afraid to blow the lights in my entire city.
I would sincerely appreciate any help.

EDIT: Ok, so minutes after posting this... I try unplugging the SD card connector from RAMPS, and viola! Everything got power! Motors and bed started moving, lights were flashing on RAMPS, birds were chirping, etc. So now I'm not sure what's going on. I don't think the bed should just be moving back and forth permanently...

Edited 2 time(s). Last edit at 11/26/2013 09:50PM by mza1979m.

If the bed and other motors are going backwards/forwards, the board could have test firmware on it, rather than something like Marlin.

Seems like the SD adapter was either on the wrong pins (shorting out the supply) or has a fault.

FYI: The way the voltage rails work is as follows: RAMPS provides (+12V) power to the Arduino Mega. The Arduino Mega has a regulator on it that provides 5V for the Arduino Mega as well as +5V power back to RAMPS. If you somehow short out your 5V rails on RAMPS, the Arduino Mega regulator will either die (smoke) or shut itself down (depends on how much of a short it is, if it's an original or clone, etc).

Take a look at the SD card adapter. Make sure there are no blobs of solder between pins. Double check where you were plugging the adapter in. It's very easy to get moved over one pin the wrong way on these boards.

The computer should power the Mega + Ramps just fine from the USB. It will not run the printer this way, but it should power up the Mega so it's recognized. It's probably safer in your case to check the SD issue out this way than with the high power supply connected.