Zhou,
Using NI-DAQ 6.8 will work fine with the PCI-6527 you currently have installed. However, it is always best to have the latest driver.
If you decide to use NI-DAQ 6.9.1 instead, then follow these steps:
1. Uninstall 6.8 that installed with LabVIEW 6.
See this KB for more detailed steps:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/08f0a1cf535d995086256ab6006bdefc?OpenDocument
2. Reboot.
3. Install NI-DAQ 6.9.1 using the CD you received. Be sure to select LabVIEW 6 support when you install.