QFileDialog::getOpenFileName() method show a disable select file dialog on Mac 10.4

Dear All, I am developing an Qt application runs on Mac 10.4 to 10.8. I use Qt 4.8.4 on Mac 10.5-10.8, but on Mac 10.4, I use Qt4.6 (because Mac 10.4 supports Qt 4.6), but when I show select file dialog by QFileDialog::getOpenFileName() method, the dialog is shown but disable. I try to create a very simple application just calls QFileDialog::getOpenFileName() to show select file dialog and it works, what I do wrong here, please kindly help me. I am new in Qt, and Mac too :D