As I understand from your question you have a dataview parameter DM_DiaPhuong, and you want to select the correct DangKyKham, with a reference selector. In the reference selector you need to select the association and an attribute you want to show to the user.
To be able to select something, there have to be records in your DangKyKham table. If thats the case, you should be able to see the values in your dropdown.
Can you share a screenshot of your page in StudioPro?
it will have a form like this with fields TinhString, HuyenString, XaString belonging to DangKyKham table so every time DM_DiaPhuong/tendiaphuong is selected I will assign it to String variables. Is there a way to make the reference selector display the value instead of using String variables?