Board Thread:Spoilers!/@comment-24029015-20140601003136/@comment-5834856-20140602082643

I don't know for sure, but personally I believed by reverting the first Dark Curse Regina send everyone, including Whale/Frankenstein, back to the Enchanted Forest.

Considering possibilities to get back to Emma  Regina explained that

Regina: There are no more portals. Not for us. Not for anyone the curse brought back.When I undid the first curse to escape Pan, to bring us here, it devided our realms. It placed a wall between them.

(EP: A Curious Thing)

The Dark Curse is the only possibility left to cross realms.