Instructions
Complete the campaign mode of the game in "Jedi Knight" difficulty. Often, completing the game in this mode alone will earn enough medals to unlock the wrist rockets item.
Complete the campaign mode in "Padawan" difficulty and then complete the first level of the campaign on "Jedi Master" difficulty. Medals rewarded in Padawan difficulty aren't worth as much as those from Jedi Knight or Jedi Master. However, the first level or Jedi Master typically gives enough that, added to a complete Padawan playthrough, the item becomes unlocked.
Complete the campaign on Padawan difficulty and then begin on Jedi Knight. Although it will take a few more levels than using Jedi Master, the medium difficulty (Jedi Knight) also adds enough medals to those earned through Padawan to unlock the item.
Unlock medals through multiplayer, conquest and instant action modes. Evey medal unlocked adds to the total points rewarded, so achieving certain medals in other game modes can help unlock the wrist rockets sooner in a campaign playthrough. Sooner or later, the weapon will be unlocked.