Well, if you are in Chalma it has to be because you want to see this temple, wheter it is for cultural or religious reasons.
The sanctuary is located in a curious place, contrary to most churches it is not on a visible place, instead it is at the bottom of a valley. The reason for this is that, as it happened to many of the oldest churches in Mexico, built during the yars after the spanish conquest, the sanctuary was used to replace a indigenous place of cult. In Chalma the inhabitants used to worship the god Oztoctéotl inside a cave that has been buried under the church, one strategy to evangelize the indians was taking the catholic cult to the places in wich they were used to worship and that happened here.
Today the temple is completely surrounded by all kind of vendors in all directions, and during your descent to it you won't see any other thing that little stores, but all that crowded space just comes clean once you enter the forecourt wich is respected by everyone. The temple is of clear colonial style and it is quite pretty on the outside.
The church by itself is not what attracts the attention of thousands of faithfuls every year, everyone is looking to ask for a miracle to the black christ, a figure of a black colored Jesus wich is said to be very miraculous. The christ you see today is not the original wich is suposed to have been made of palm leafs, while the legend says it appeared by itself in the old cave, wich led to the construction of the sanctuary, it is most likely that it had been put there by the missionaires to induce people's faith, the original christ would have been later replaced by the more perdurable statue that we see today.
It is an interesting visit, and it will be even more interesting if you have the opportunity to be here during a mass in the christian holy week, when pilgrims come from all over the place, it's interesting to see so many people joined by their faith.
The other option to reach Chalma is joining a group of pilgrims. The sanctuary is visited by catholics all year, and its usual that groups of people get organized to visit the place, they go all the way there walking following the routes that cross the Ajusco mountains at the south of Mexico City. I have never joined a pilgrimage of any kind so I can't really give you much information, but for what I know the walk there would take you between 2 or 3 days. I suppose it would be good to speak spanish if you are interested in joining a group, I've never heard of foreigners joining the pilgrimage, so it can be quite an adventure for you.
Arriving to Chalma is quite easy, as I told you it is a very important pilgrimage destination fro mexican catholics, and that means a lot of people travel there every day. As a consequence bus rides are constant. Starting at 6:00 A.M. and during all day there is a bus that will take you to Chalma every 20 minutes; tickets do not even mark a specific schedule for your trip and you can climb on the bus departing when it suits you better. Some people even stops the bus in the middle of the street and climb up paying the ticket to the driver if there are available places.
The trip to Chalma will take you a little less than two hours and you can either visit the place and stay there or go back to Mexico City the same day.
There are four different bus stations in Mexico City, one in each cardinal point, the transport to Chalma only departs from the western station located outside the Observatorio subway station, the company that manages the route is "Autobuses Águila" and the cost of the ticket is $80 mexican pesos.
Once in Chalma the place is very small and you can walk to the sanctuary from the station without much trouble, as you can see you'll find no difficulties at all to visit the place.
The other place to take buses to this town is Toluca, I can't give you much information about the options from there, but I think it is also an easy trip.