Firstly, whether or not atheists have meaning in their lives is irrelevant to the question of the existence of a god. Secondly, of course atheists have meaning in their lives.
It's important that we establish that, since there is no god, our lives do not have an externally-determined grand design, plan or divine purpose in the sense that theists often mean when they use the word "meaning". The same holds true for theists; since there is probably no god, their belief that their lives have this kind of meaning is a delusion.
The only externally-determined meaning our lives have is a Darwinian one. We, like all other organisms, were designed by the non-random process of evolution to do one thing only: to survive long enough to reproduce and pass on our genes to the next generation. That is the only externally-imposed meaning we have, and not all of us either choose to, or are able to, reproduce. So does that mean that child-free people are without meaning in their lives? Of course not. It simply means that any meaning or purpose beyond the strictly Darwinian purpose of reproduction is determined not by some external power, but by myself. Just as I am responsible for my own actions, I am also responsible for deciding and determining the meaning and purpose of my own life. There are no gods to do it for me.
The idea of a life given meaning by a god may be comforting to theists, but to me it just sounds like an abdication of responsibility for one's own life, for one's own actions. It is the life of a slave. As a free man, responsible to no higher power, I create my own meaning.
My life has meaning if I make someone else's life a little better. My life has meaning if I am a good husband, because it has meaning to my wife. I give my life meaning by being a good friend, by making people think, by passing on knowledge, or any of a thousand other things. My life has meaning in as much as I touch the lives of others.
If I can leave the world as a slightly better place than it was when I came here, even if only for a few people, then my life will have had purpose and meaning.
It's also worth noting that since I take responsibility for my own actions and my own meaning, rather than abdicating them, in favour of an imaginary meaning imposed by a god, I can also take the credit for any good things I do because it was me that did them, not a god working through me.
So what is the meaning of life? I've described what it is for me. For you it may well be different. Whatever the meaning and purpose of your life are, though, it is you who have the responsibility for creating it. There are no gods to do it for you.