The current Create Datasource dialog has a few usability "gotchas":
1) It is located under the "JBoss Tools Web" category - when it is more appropriate to be under the "JBoss Tools" category, closer to the Forge wizards - where you need it.
2) It defaults to AS5, not AS7 - it should default to EAP6/AS7
3) It defaults to register in persistence.xml - it is unclear to me how cleanly this injects into the current persistence.xml provided by the Forge wizards, I tend to uncheck it to be safe - then manually tweak up the persistence.xml to reference the same resource.