Did you name your platform PSX? By default in the associated platform tab for RetroArch it will automatically setup a line that says Sony Playstation -L "cores\mednafen_psx_libretro.dll" if you did name your platform PSX you will have to change the 1st part of that line so it matches your platform name so example: PSX -L "cores\mednafen_psx_libretro.dll". If that isn't the issue will need some more info.
I should have asked 1st but can you get the games to run just using RetroArch without LB? Do you have the proper BIOS files if it doesn't work through RA I'm guessing you need the BIOS files or you have bad rips