The plugin needs to be installed to the engine via Epic Launcher (not from Fab website or Fab plugin in Engine)… however, I just checked to see if I could add it to a fresh install of UE5.4 and it was not appearing for me in the Launcher’s Fab library.
I’ve posted a report of this to the “Issues and Bug Reporting” board here ( Metahuman Plugin Not visible in Fab Library/Vault ), so hopefully the glitch is found and addressed by the team soon.