By not voting you are just agreeing with the majority of those who voted. So yes, by not voting you are endorsing whoever wins the election.
Not true. By not voting, you are stating that _neither_ candidate or party represents your interests (or, if you are an anarchist like me, that the whole system runs counter to what you believe in). You are no more responsible for the outcome than you would be for refusing to answer a sadist who asked you to choose which of two strangers he should murder.
Look, I know that this goes against everything they taught you on MTV's Rock the Vote ("Choose or Lose" bus, anyone?), but you have bought into a brilliantly engineered false dilemma that perpetuates the system as it stands.
"Any excuse will serve a tyrant." -- Aesop