How do you unlock Stardust Dragon in World Championship Mode in Yu-Gi-Oh 5Ds Stardust Accelerator

If you are talking about unlocking the duelist, there is no "Stardust Dragon" duelist. There is a "Stardust Dragon/Assault Mode" duelist who you unlock by summoning "Stardust Dragon/Assault Mode" in a duel. If you want to know where to find the card, it's in the The Duelist Genesis booster pack, which you unlock by defeating the following duelist three times in story mode: Barclay, Shuji, Taniyama, Ellison Noula, Skathi, Yagishita, Geruzet, Motoi, Kyono, Fudagawa, Dio, and Vega.