The Dragon Arena itself is located beneath the red obelisk (in single player, if you use the 'GHOST' command and float up, it will actually put you below the green obelisk), but the portal to the arena can be opened from any obelisk or Supply Crate unless playing in Single Player mode, in which case the portal can only be generated at the red obelisk.
The following items are needed to summon the Dragon:
These restrictions do not apply to Extinction arena as the boss fight takes place inside the map rather than in a separate section of the map
A maximum of 10 survivors and 20 creatures are allowed to enter the arena. In the case of Tek Cave, a maximum of 10 survivors and 40 creatures are allowed, but only 20 creatures can be warped in at any point of time to the arena itself per warp.
Most Arenas restrict survivors to ground-only creature. There are a few exceptions:
Genesis - Sea Creatures are permitted in Moeder arena, and its drag weights are not considered when warping to the arena.
Exceptions for certain arenas are only valid for the listed arenas. E.g. Manticore Arena refers to the Manticore Arena in Scorched Earth but not Ragnarok Arena where you also have to fight against the Manticore.
A creature's "drag weight" must be less than 560. This is how much the creature itself weighs, not its carrying weight seen in its inventory.
To see this value in-game, place the creature on an Elevator Platform with an empty inventory and no saddle.