Enterprise Adapters were originally designed to work with Broker (then called webMethods Enterprise Server). Back in the IS 6.01 release, the adapter run-time (ART) was moved into the Integration Server. IS has never supported Enterprise Adapters, but you could still use them as Broker clients configured with some Broker admin tool (Broker Manager?).
For the last 4-5 years all adapter development has been targeted for the IS ART and most Enterprise adapters are nearing end of support if not already there.