Instructions
Play through the game until you get to Chapter 6, when Spinda's Cafe opens. Go to the crossroads between the beach, Treasure Town and Wigglytuff's Guild to find the cafe. Enter and get the tour from Spinda.
Speak with the Pokemon standing to the left of the exit inside of the cafe. He will occasionally have jobs for you. This, like many aspects of the game, is completely randomized, so if he doesn't have a job for you, ask him again the next day.
Play through the game until you have beaten Dialga, the final boss of the main storyline, and graduated from the Wigglytuff Guild. Do any missions you haven't done yet and check the Kecleon Shop near the Wigglytuff Guild every day. Eventually, a Mr. Mime will appear with a new mission for you.
Accept Mr. Mime's mission to rescue Scizor from Froslass. Follow Mr. Mime's directions to Blizzard Island and fight your way through the dungeon. Defeat Froslass and Scizor will be free. He rewards you by marking your Guild Card with a secret rank.
Return to Spinda's Cafe once you have the secret rank and speak with the Pokemon standing near the exit to get new missions.