A lot of existentialists are misinterpreted as being nihilistic when, in fact, that is one of the things that they actually stand against. This comes back to the idea of readings of a text and you can certainly do a nihilistic reading of an existentialist text. While this may be the case I think it would be a mistake to equate existentialism and nihilism to mean the same thing.
A lot of existentialists start with the nihilistic concept that there is no meaning, that there is no inherent meaning in human existence (something which comes out stronger from an aetheistic stand point). While they may start here, many of them will then seek a way to find a certain sense of meaning within this meaninglessness. An example is Albert Camus when he looks at the futile labours of Sisyphus (greek dude rolling the boulder up a hill) yet manages to conclude that "sisyphus must be happy" given the powerful moments when he triumphs over the absurdity of his situation.
Taking another example, Nietzsche. He viewed christianity as being nihilistic. If you ever read much of his stuff you will soon notice that he isn't exactly the biggest supporter of christianity and a big reason for this is the nihilistic tendencies that he percieves in the religion [obsessing over the after life while they abstain from living life to the fullest. In this rejection of the worldly he sees nihilism].
I hope those general statments help you out. In the end it's my personal view that while nihilism is a prominent subject for many of the existentialists, this does not make nihilism and existentialism the same thing.