Have you beaten level 9 already?
Also, what version of ZC are you using? If you're using anything other than version 1.90, I'm sorry to say that you cannot enter the bonus dungeon afaik. There is a bug in later versions of ZC that prevents you from getting the item that allows you to enter the bonus level in the U3Q.
I could tell you where it is, but it is cleverly hidden, and finding it on your own is very rewarding. (I was surprised when I guessed correctly where it was)