Above, you will see a map with a detailed view of the entrance to the Hacienda. You will find it at kilometer 68 in the freeway - there will be an archway marking the spot. Taxis will be notified of the event to ensure they will be easily accessible to and from the venue. There will also be a parking area in the venue, but the cars must leave at the end of the event - they cannot stay overnight.
There is a lot of traffic in the freeway leading to the city, so make sure you plan ahead if you're trying to make a reservation or if you need to leave for the airport.
Here is a great link with a guide of the downtown area. https://bbqboy.net/a-photographic-walking-tour-of-san-miguel-de-allende-mexico/