Mahmoud's is the type of place that once you've been there it is a snap to find but it can be a little difficult the first time (simply because the souk is bustling and traders will be trying to entice you into their own shop).
Luckily though...Meissoun has done the hard work for you and gives you directions and photos to make it easier to identify - the webpage is
Cairo - Adresses for Shopping
You are going to love Egypt! you lucky duck, wish i was going with you