You need to check the relevant data sheets, and be more forthcoming on your code and equipment.
My 400x240 has the SD on the shield, and I had to bridge a pair of solder pads in order to use it. I understand yours is on the display but you may have to do a similar trick somewhere, most likely on the shield. I also had to change the chip select from the usual 4 to 53. This is in addition to the usual command
required on the Mega. I believe both commands relevant to pin53 are required. This applies to the standard SD CardInfo example.