Workshop support is added by the Afterbirth+ expansion. Workshop mods can therefore only be used on Afterbirth+, Repentance, and Repentance+.

If you're on Rebirth or Afterbirth, forget about the workshop and lua mods entirely. Check moddingofisaac.com instead. Don't expect too much from Rebirth/AB mods though, modding is quite limited on those versions.