ramps 1.4

so basically i wired up the ramps 1.4 flashed it with marlin but when i plugged it into 12v nothing come up but when i pluged it in via the usb it shows up on the lcd

I don't really understand your question, can you be more specific?

the diode D1 in the ramp isn't installed and is not supplying voltage to the mega board

it is on there here is a pic of it

Sounds like it may be a dead voltage regulator on the mega then

Unplug the mega from everything

Plug the mega into the computer via usb, upload the blink test, make sure that works
now unplug the usb cable. Plug in a dc power supply into the dc jack. (7-12V)

Does it turn on and run blink?

I had a similar problem on my printer. USB plugged in, worked fine, 12V plugged in only, did not work. I had fried my 5V regulator on the Mega 2560 board so it only worked on USB 5v power.
This happened when I wired my endstops to my RAMPS. My cable was wrongly wired and I shorted my 5v to ground on power up. (let the smoke out)
I was using an old computer ATX power supply which supplies +12V and +5V DC (along with others)
My initial solution was to use the 5v off my ATX power supply and an old USB connector wired so I could plug it into the USB jack and not need a PC when I was mobile using an SD card.
It would also work when I plugged in my USB from the computer.
I eventually just bought another Arduino Mega and I don't have that issue anymore, but I saved the old one as a backup (just in case).

This link has some interesting info on the Arduino troubleshooting.
[arduino.stackexchange.com]

i did the blink test and it worked

It's hard to tell from one photo, but it looks like you might have a bad solder joint at the negative end of the diode. Try resoldering that joint.