The EMU does not find the .bin files from just "Load ROM" you have to go into the Atari blue-screen to load them. And the same appears for A52 files. (well, the A52 files will show up, but not load, that way...you still have to load THOSE from the Atari bluescreen)
I don't know about .ATR files...I don't have any.
So, it would seem, with this EMU, you always have to load from the bluescreen. In which case, I have a convenient way to always get it right when loading carts...rename the cart to include what kind of cart it is, after the game name, for example Berzerk 2Chip.A52, Defender 1Chip.A52 etc.
Mostly, now, this EMU is working out better, but, some games do not work with the key configuration, and I can't seem to figure how you set them for a single game. Moon Patrol, for example, comes to mind...you have a fire button, but no jump button...not good!! You need both to play that game. Likewise, Pole Position, there does not appear to be a button which acts as your shifter, so you are stuck in only low gear.
Also, still can't make Defender and a few others like it...to actually start, they have like, an option screen I can't seem to get past.
A few other games do not work out well, because of the controller itself, Missile Command comes to mind here, you can't accurately control the crosshairs. also, Gorf is bad, because you can only show up on left edge, right edge, or dead middle, it does not "smooth-scroll" move your shooter...and, if you move to an edge, and then let go, it snaps you back to center...very tough to line up shots this way. But this is likely the ROM and not the fault of the EMU.
I'd sure like to figure how to get Defender working....how to configure buttons for individual games, so I could play games like Moon Patrol or Pole Position.
Some games I'm going to get rid of, just because they aren't really playable...like Missile Command and Gorf, are, in my opinion, unplayable - probably due to faults with the ROM itself and not the EMU.
This EMU is now working out much better, now that I know how to load games and we have a correct key configuration so that games can be started. Still, there's room for improvement here. And the README file really needs to get updated to reflect how you do all this stuff.