Expressing an expression w/ NAND/NOR:

First, the thing to realize is that you may use nots, because NOT(A) = NOR(A,A) = NAND(A,A)

So, A + B expressed in NAND is:

NAND(NOT(A),NOT(B)), because NAND(A,B) is NOT(A) + NOT(B).

A + B expressed in NOR is:

NOT(NOR(A,B)), that is essentially NOT(NOT(OR(A,B))), so the NOTs cancel out and you get A + B