This answer was correct when it was first posted, but the instructions no longer work (confirmed by several other users in the comments on this answer). In fact, I am not sure what the current solution is, or even if there is a solution.
I want to ask a new question: "How can I sync multiple calendars from my google account to windows phone?" But this seems like a duplicate question, even though the original has an incorrect answer.
What should I do in this situation?