Alright! i got the game yesterday and my Hunter is on LVL 11 at the moment. I checked the official site and I found this:

"The Expansion Pack needs to be activated in-game after purchase, and this can only occur once the main story has been completed."

and this!:

"Players who have purchased the Origin Island Expansion Pack cannot connect in multiplayer with players who have not purchased it. Multiplayer connection will only be possible with other owners of the Expansion Pack."

So if you are going to play online, make sure everyone has bought the pack - or NOT have bought it already, ok? I haven't bought the pack yet, I will beat the main story first.