How BCD Ader shows the correct BCD number?
Well you said it yourself, 18 in BCD is 00011000. When doing the BCD correction you will find that that first 1 in 000011000 is actually a carry. Ignacio basically said the same thing, but that first 1 is the carry, and the remaining bits (1000) are S8S4S2S1, so you get 11000 as CoutS8S4S2S1. The leading 0's don't matter.