Hi,
The 6608 must be in slot 2 of the chassis. Once you make a call to the DAQ driver, the OCXO on the 6608 will be routed to the PXI_Clk 10 line.
More information on the backplane and routing clock signals can be found in these Knowledge Bases:
http://digital.ni.com/public.nsf/websearch/5EC370419A5ECA7A86256CFC0061C528?OpenDocument
http://digital.ni.com/public.nsf/websearch/D5B8D4D3B67DF1E086256BF8007BBF93?OpenDocument
I hope this helps. Have a Great Day!
George