Thank you for your answer, it works!
I am not using Mobipocket Creator since I am on Mac. The way I do it is like this:
1. First I make epub for iBooks.
2. Then I adjust the files for Kindle.
3. Make a new epub. (After I add the <mbp:nu> into my files, the epub does not validate anymore, but I guess that is okay
)
4. Convert it to mobi using KindleGen.