Atari St on Retroarch

Hi everyone, I am new to the forum.

I hope someone can make sense of this please, as I feel I have reached the end of my sanity with this.

I would like to install the Atari ST emulator on Retroarch.

I have downloaded the only core that is on there (Hatari).

My understanding is that you need a TOS.img file and a .cfg file.

I have the TOS.img file, and I have put it in the Retroarch systems folder.

However, when I navigate to the settings menu within Retroarch, then go to directory / system/BIOS settings, Retroarch does not see the TOS.img file. It just says "No Items" etc. So i can't select it.

Can someone please help me understand why Retroarch is not seeing the TOS.img file?

I'm also lead to believe that I need to put the .cfg file somewhere to, but i cannot find this file within the Retroarch folders. Am I looking in the wrong place?

Just for clarity, I have mangaged to get the Hatari emulator to work as a standalone, and obviously I have this installed seperately, but I would like to use it within Retroarch.

In the hope that someone can help me get passed this point, can I also ask: What format should the ROM files be in?

I am working on a Macbook if that makes any difference.

Thank you in advanced for your time.