.map files are the "source" file to the map, dpball2 can't run that file. You need to open the .map file using the BSP map editor, and then compile the map into a playable .bsp map. To run the map you put it in you /pball/maps/ folder, and can then run it from console (~) using "map <mapname>".
Since the map will be a beta, name it something like <mapname>_bv1.bsp, as in "beta version 1", just add something like that to the end so that you don't have a bunch of versions of the map laying around with the same name in different folders.