I know, it’s needlessly confusing. I say just add Emulation Station and play most ROMs through that. Here’s how (from memory):
- Open Steam ROM Manager
- on the left-hand list enable the “Emulators” one
- then click the top “Preview” link near the top of the sidebar
- Click “Generate App List”
- It should add a bunch of emulators, make sure Emulation Station is in the list
- Save so it stores it in Steam
- Then, go into steam
- delete the other emulator entries because you only care about ES
- Launch Emulation Station, and your ROMs should be here
HTH