Its a storyline only mission. During the storyline you get a set of waypoints to get through the mines (i think). After the storyline the jumpgate is locked. However there is a mod that opens it.

You can actually find the entrance into Zone 21 by looking at the patrol paths and doing a bit of interpolation. However, as redeye has pointed out, the gate is always locked except during mission 11.

So, to get to Alaska you need to start a new game and follow Juni's directions for about five hours.

You can get in in single player, or if you decide to use Reynen's Mod 2.0 Beta 5, you can access it. It's just like single player, you can't dock at Juneao (excuse spelling) Shipyard, and when you follow the buoys to Mitchell Prison Station, and dock there, the screen goes blank except for your neural net and the only option at that station, Undock. If you wanna check it out, turn on Reynen's mod, make a new character in multiplayer, I've done this with one of the chinese servers so they can't yell at you because you have a mod on, ANYWAYS, then go to the Alaska gate and dock, you go there, after that, its just fun to look around.

Btw, Reynen's mod rules.

yes the gate is locked...but i managed to try and bully myself thru the minefield with my battleship. Ended up with a floating heap of superheated ship panels and scrap metal *sigh*