Hi @Nasseer,
In 3.4 the DOIs are not in a plugin any more, they are in the core, so you do not need any plugin for DOI assignments any more. You only need a registration agency plugin (e.g. Crossref), that is now under the generic plugin categery, to register your DOIs.
EDIT: DOI settings you will find under Distribution > DOIs.
Best,
Bozana