How can I get to the Atlanta airport from the Proscencius bldg where AMA is located using MARTA?

I would suggest use your GPS and set destination to reach Arts center station. Once you reach the transit station load/reload your breeze card with Atleast one trip (costs 2.50 $ each trip + 2$ breeze card fee). You can obtain the breeze card from any of the vending machine. Get to platform level and catch onto SOUTHBOUND train. Get off the train at the last stop, In about 30 min you can reach Atlanta domestic terminal(expected of no delays in schedule)

Yep, southbound train straight to the airport in about half hour.

Take the train southbound to the end of the line. It takes you right into the airport