The following wrestlers can be unlocked by either buying them at the Smackdown Mall or by completing the following tasks:

Jerry Lawler: Defeat the 21st opponent in survival mode.
Shane McMahon: Defeat the 31st opponent in survival mode.
Mae Young: Defeat the 41st opponent in survival mode.
Cactus Jack: Defeat the 51st opponent in survival mode.
Ken Shamrock: Defeat the 61st opponent in survival mode.
Mick Foley: Defeat the 71st opponent in survival mode.
Vince McMahon: Defeat the 81st opponent in survival mode.
Shawn Micheals: Defeat the 91st opponent in survival mode.
Andre The Giant: Defeat the 101st opponent in survival mode.
Alternately, the following characters can be unlocked by playing through championship mode:

Vince McMahon: WWF course, chapter 10-3
Shane McMahon: Intercontinental course, chapter 9-4
Linda McMahon: WWF course, chapter 10-2
Earl Hebner: WWF course, chapter 10-4
Shawn Micheals: WWF course, chapter 10-9
Cactus Jack: Hardcore course, chapter 9-7
Mick Foley: WWF course, chapter 10-1
Andre The Giant: WWF course, chapter 10-8