The line down the centre of each symbol is a line of symmetry. On the right hand side of this line, a digit is drawn. The left hand side is therefore a mirror image of the digit. The first is 1, second 2, third 3 etc. Therefore the missing digit is 6 and so the missing symbol is:

The second code is based on the same idea, but this time the letters of the alphabet (capitals) are reflected in the central line of each symbol. The message reads "DECODE THIS".