Double check the polarity of C6 and C1. If it is wrong they will explode!
Start with U2 not fitted and just the power supply connected.
Turn the power supply on. The LED should light up and the voltage between pins 1 and 12 of the 12 pin power connector should be 12V +/- 10%.
If it is 0 and the power supply definitely has a good mains supply then see http://reprap.org/bin/view/Main/PCPowerSupply, your PSU probably needs a link to make it power up. Disconnect the PSU and test it separately to rule out a short circuit on the powercomms board.
If it is around 5V then the 4 pin Molex connector is reversed. The yellow wire should be to the corner of the board.
If the 12V rail is too low, or it is pulsing, then the PSU does not have enough load to regulate properly. Provided it is above 7V the powercomms board will work but the rest of the machine may not. Try wiring a 4.7R 5W resistor, or a car sidelight bulb, across the 5V rail of the PSU to bring the 12V rail in range.
If the 12V rail is OK but the LED does not light check the orientation of the LED and the value of R1.
Check the voltage between pins 15 and 16 of the socket for U2 is 5V +/- 5%. If it is wrong check the orientation of U1 and C1 and look for bad joints or solder shorts around those components.
Connect the RS232 cable from the board to the PC. Check that the voltage on pin 14 of the socket for U2 is around 0V and the voltage on pin 13 between -5V and -12V.
If these are reversed you probably have a cable which swaps pins 2 and 3, you need a straight through cable with at least pins 2,3 and 5 connected.
Otherwise suspect a faulty cable or PC serial port. Try measuring at the pins of the RS232 connector with the board disconnected to rule out shorts on the board.
Run Hyper Terminal, or your favourite terminal emulator, configured to the serial port you are using and 19200 baud, 8 bits, no parity, one stop bit, no flow control or handshaking and no auto echo.
Check that with a link between pins 13 and 14 of U2's socket characters are echoed and without a link they are not.
If you get a double echo, or echo with no link you probably have auto echo on in your terminal emulator or a short in your serial cable or on the board.
If you get nothing try linking pins 2 and 3 of the of the connector on the back of the PC. If this fails either the serial port is faulty or it is not the same port number you specified in the terminal emulator.
Otherwise suspect a faulty cable.
Switch off, remove the DB9 connector and insert U2 into its socket, checking orientation.
Switch on again and check the following voltage on the pins of U2 with the meter negative lead on pin 15 or any other 0V connection you find convenient.
Pin 15: +5V +/- 5%
Pin 2: around +9V
Pin 6,7 & 14: around -9V
Pin 8 & 13: around 0V
Pin 9 & 12: close to 5V
If any of these are absent or wildly different check the orientation of the MAX chip and C2, C3, C4 & C5 and look for for shorts or opens around these components.
Reconnect the RS323 cable and check for echo with pins 2 of P2 and P3 linked and no echo without.