Select "Import Content" -> "Scan Directory" in RetroArch. Select your ROM folder.
Parent ROMs and Child (clone) ROMs are separated. To play a clone (e.g., Street Fighter II: Champion Edition), you need both the Parent file and the Clone file in your folder. Mame 2003-plus Reference Full Non-merged Romsets Download
A true reference set will often include an explicit .dat or .xml file matching the MAME 2003-Plus build. Select "Import Content" -> "Scan Directory" in RetroArch
Instead of hunting shady downloads, Marco: Street Fighter II: Champion Edition)