One is; how is chance quick? And the other one is; what powers the mutations in the right direction?
1) Chance is not quick, as we've spoken about, it has taken hundreds of millions of years for species to evolve to their current state. And even the Cambrian Explosion took place over 5 million years. So by any stretch of the imagination, Evolution is not quick. And really the separate answer to this ties in with your second question.
2) Ability to survive. Simple as. There is no unseen power which gives animals 'good and bad' mutations. All humans are born with these changes in their genes, which explains why you do not look exactly like your parents. That in itself is a change. It is the ability to survive, however, which ensures that those with bad mutations die off, and those with beneficial mutations survive longer, reproduce more etc.
EDIT: I should note though, that evolution is not a ladder. Humans were not the obvious result of all these years of evolution. It is considered a tree, with species branching off in different directions. Evolution is mindless, in that sense. It is the external factors that dictate which species live and which do not.