Hi ming2008,
I faced the same problem, first time I tried to use the .
but all the fields are there trust me - I assume that you choose the right table.
Here the IMPORTANT thing,
There is a CheckBox at the beginning of the wizard said "Do you like to show the real name of the table and fields" or something like that- actually I don't remember exactly since I don't have DAX right now).
Then when you click next you will see the real name of the tables and fields, and hopefully you will find your missed field.
In fact, the default representation of the fields in Report Builder is the label, so some time the name is completly differnt that why (as I think) you can't find the field.
I hope this is the case and I helped you to solve your problem.