Hi,
There's a few options for traveling to Mar del Plata from Buenos Aires:
Car or coach:
Motorway 2: 404 km / 251 miles
Route 11: 602 km / 374 miles
Route 226: 441 / 274 miles
Rail: 5 hours 30 minutes
Plane: 45 minutes
One options for airlines is:
http://www.aerolineas.com.ar/