If the access point has an 'E' at the end, it means external. For example MR42E, which allows you to use an external antenna (directional for example). Otherwise, all of the antenna within the Meraki lineup are internal omnidirectional (gain might vary per model). You can find their antenna pattern and gain information on the datasheet for the specific model your looking at.
For the MR series the antenna patterns are in the datasheets. As @NolanHerring said, for -E access points it'll be in the datasheet of the antenna.
For the MX I haven't seen antenna patterns yet, but I'd expect it to be more like a somewhat flattened sphere. In the up and down directions the signal will likely travel less far (assuming the antenna's are pointing upwards).