If it's your first time playing a CDLC and if you use Windows, you should download and paste the file D3DX9_42.dll into the root folder of the game. If you use MacOS, you must install MAC_PATCH. I think you should also have or buy the cherubrock_p.psarc. I downloaded the song Metallica_Cure_v2_NDD_p.psarc and it worked without problems.
If you only need the RS Custom Song Toolkit, this version that I leave below worked perfectly for me. RS Custom Song Toolkit v2.9.0.0-b1c03443 This information was obtained from the post of I)ark_Seph
When I run the Mac_Patch (RUN_PATCH_RS.command) in the terminal, all the songs in the RS1 disappear, I can't play them. Could you check your game and confirm me, please? To play songs from the RS1 again, I must uninstall the game and reinstall everything again, except the mac_patch.
I never had problems with High Sierra. Now I use MacOS Mojave 10.14.2 and I have no problems either. You should be more specific and at what point it crashes.