You need the Homebrew App Store, which you should already have. Retroarch is on there!

it says that its only for legacy...

Yeah I tried retroarch on aroma from github and it gave me that scary error that looks like the one you get when the nand dies. It was only with retroarch so I uninstalled that.

Oh yeah. Apparently you need Tiramisu to use it. In that case, I would recommend standalone emulators (like Snes9x GX or VBAGX).