If you did not download the motherboard manual you really need to do that and look at the diagrams there. It does explain which are the positive and negative terminals.
Now you will have to go thru a step by step process to find out where your problem is.
Likely Cause: Could be anything.
Determining whether the issue stems from hardware or software is part of the fix.
The Fix: You'll have to play Sherlock Holmes to figure out what's dead. Take it step-by-step.
1. First thing: Check all cables (including the plug into the electric socket) to make sure everything is hooked up nice and tight. You need to double check to see that all cables and wires are hooked up properly and to the right connectors.
2. Next, see if the power supply turns on. Listen for the sound of its fan or of your hard drive spinning. If you hear nothing, your power supply probably needs to be replaced. To confirm, consider testing the voltage output with a power-supply tester such as PC Power and Cooling's $10 ATX). Of course, you should also check your home's circuit breaker before doing major PC surgery, and try powering another device from that outlet to make sure it's getting juice.
3. If your power supply is okay but nothing appears on screen, plug in a different monitor (borrow one if you must) to ensure it's not your display that's blown. If the monitor proves to be good, try replacing the video cable. Still nothing on screen? If your drive is spinning normally, your video card is probably bad. To replace it, see our video guide, "How to Replace a Graphics Board," or use the video output integrated into your PC's motherboard, if it has that feature. While your case is open, make sure all the fans inside work when you power on the PC. You could have excess-heat issues.
4. If your monitor is working but you detect no hard-drive activity and see no display (or you see a display but the PC can't get through boot-up), reset the CMOS. Shut down the PC, unplug it, ground yourself, and take out the battery on the motherboard (click on photo above). Wait 5 minutes, and consult your PC manual or go to the vendor's Web site for instructions on resetting the CMOS jumpers. Reboot and see if that fixed the problem.
5. If the PC is still not functioning, bad RAM could be the culprit. Remove one memory module at a time (or replace each module with a known good one) and reboot after each test. Alternatively, create a free MemTest86 boot disk on another PC, and try using it to test the RAM.
6. If none of this works, your motherboard or CPU is probably damaged, and will need to be replaced. It may be that your Motherboard or a hardware was bad from the get-go. See if you can RMA (Return Merchandise Authorization) any hardware that is bad, back to where you bought the hardware. The problem here is you may have wired it up wrong and caused the problem yourself. They may still replace it but the also may choose not to do so.
7. Finally, if the PC's BIOS routine runs but the drive won't spin, your drive may have crashed. See "Problem: Your hard drive has crashed" for help with that.
I do not understand what you mean by "case connector is 2 pin and the motherboard is 3..". The Manual clearly shows only 2 pins are needed for the "power" switch. If you are talking about the "power on" light, you really don't even need it right now. It does not affect anything in the way the computer boots up. It is only there to let you know the computer is turned on. Any good local computer shop should be able to switch a 3 pin connector to a 2 pin connector if that is what needs to be done. The only thing a 3 pin connector type light does is have a green light for power on and a amber light for problem startups. Your motherboard does not even show this type of pin setup on the motherboard. It only shows a 2 pin setup which are the 3rd and 4th pins on the top row of the front panel pin set.