Thanks Sd-Snatcher. 
I connected the external power supply and now the slot expander is alive. It's able to recognize my Snatcher Sound Cart, MegaFlashRom SCC, and FM-Pac in the same slot without any problems. The only cartridge that doesn't work in this slot expander is my Padial 4MB Ram, it needs to be in an unexpanded slot. Is there a reason for this?
So it has no use for me as I don't have a Megaram! 
Anyway, thank you very much!
It doesn't need to be a megaram. It can be a mega-flash-rom or any other megarom emulation device.
In fact, the switch is used to configure two slots as they're a single one. It's more usefull for the megarom+SCC idea, but you can use that jumper for other purposes, like developing a new ROM for a memory-mapped diskdrive interface, i.e.
