I'm so stupid that I don't see where is my balance?

You can see your stats in "My stats" below the graphic (right side).

And how do i withdraw?

Once you reach the minimum balance for withdrawal, it is then triggered automatically. No manual action is required.

I have the same problem, can anyone simply explain?

It's actually rather simple, as you "mine" your address will gain shares based off of how much work(hashrate) you contribute to the pool. Once a block is discovered and confirmed your balance will be updated  based off the PPLNS scheme which you can read more about in the FAQ. This balance is what the pool owes you. However, you won't see these funds transferred to the paid out section(essentially transferred to your personal wallet) until you cross the minimum payout requirements. In the case of Monacoin, you need to gain a minimum of .5 Monacoin.

By the way, I've just deployed a bug fix regarding the stats display for new miners.