The mainboard fan was first included with the second batch of Mini Delta's and all future versions.
The fan kit mentioned below is not currently available so the linked page may not be active.
If you do have a MP Mini Delta that came without the fan in the bottom you can purchase this kit to add a cooling fan to help keep the electronics cool.
GigDigit - 6010 Fan Kit - MP Mini Delta
The kit includes:
The Y-splitter is for connecting the new fan and the hotend fan to the hotend fan header on the mainboard.
What's this oddly included sheet of sandpaper for? The sandpaper is for optionally sanding the vent holes on the base plate. The vent holes may have rough edges the stick up enough that if there is any warping in the bottom plate the fan blades could hit the metal protrusions.
The included washers are also optional. They can be used to add small gap between the bottom plate and the fan blades instead of sanding or if sanding did not work. The downside to using the washers is that you may notice more wind noise than if the fan was mounted directly to the bottom plate.