I don't know what's wrong with your code, but for comparison purposes, others have implemented SPI using a DAQ card:
http://www.ni.com/example/31163/en/
http://forums.ni.com/t5/Digital-I-O/Implementing-I2C-or-SPI-with-Pxi6508-in-Labview/td-p/554383 (scroll through the thread a bit)