Geeetech

I have some problem with the bed , it is no flat enough, is more concave both the hot bed and the aluminium plate. that is mean even if I calibrate the four corner at the same distance from the extruder, in the center the distance is bigger .

Can someone help me with this?

I get a problem with the X axis move in the opposite direction, I follow the instruction shown here ,but i found a
issue I can not found the folder mention in the step 6. Open folder "I3_Sanguinololu"(files of Marlin firmware), select all the files (59 in all) and drag them into arduino.

I ...