From what I have read, Memory Link will appear as an option on the Start Menu. It will be there when the game is first turned on. There are no sources that say "You must beat the Elite 4." or "You must talk to So&So."
However, to use it, you must have a copy of Pokemon Black Version or Pokemon White Version with the storyline already done (beat the Elite Four, Champion, then let the credits roll.)
Then, you link the 2 games via Wi-Fi or DS Wireless.
If 2 DS: have BW2 in one DS and select the Unova Link option, while the DS with BW selects Download Play. Then you go from there.
Unova Link appears on the start Menu when the game is first turned on. I have my copy to prove it.