That is clearly described in the file "usage.txt" of the FMCB release package.
Edit: Using uLE, I can navigate to mc0 only because its at the top. I see a BOOT folder. Apparently something is copied, or do all MC's have a BOOT folder already? When I installed FMCB, I had put uLE as BOOT.ELF in the BOOT folder, and so I think it would have copied or something.
That does not match my results. I've installed FMCB v1.5 both on an 8MB Sony original MC and on a 32MB Datel Max Memory MC, and both those MCs work fine with FMCB booting. When going to the Sony browser (with the added entry for FMCB) I can inspect the MC contents as usual, with all gamesave and homebrew folders being displayed as the usual icons. And if I back out from the MC browser and use the FMCB entry that will relaunch uLE.
I can't access my MC in Browser. I see it, but when I press X, nothing happens. But gamesaves are still there, I tested a game, and still have my profiles. I bought a new MC (Sony 8 MB) at gamestop and tried it, same thing :(.