我是这样用的：

//...
var cookieParser = require('cookie-parser');
var session = require('express-session');
//...

app.use(cookieParser());
app.use(session({
    key: 'session',
    secret: 'keyboardcat',
    store: new MongoStore({ mongooseConnection: mongoose.connection })
}));

//...

router.get('/login', function(req, res, next) {
    // 读取 cookie
    var user = req.session.user;
    if (user) {
        return res.redirect('/');
    }
    //..
});

router.post('/login', function(req, res, next) {
    //...
    // 设置 cookie
    req.session.user = user;
    return res.redirect('/');
});

express实例

demo:http://www.hubwiz.com/course/549a704f88dba0136c371703/,这是个nodejs+mongodb的实战的demo，里面有session的设置。