Ricardo,
You will be able to find the mapping from 68 to 50 pins within the following KB:
http://digital.ni.com/public.nsf/websearch/9B8A3D3DF8F1068D8625684700723D2A?OpenDocument
Once you understand the pin number mappings you should be able to apply the information found in the 5401 Manual to determine which pins you need on the 68 pin connector.
Hope this helps.
Regards,
Kevin R
Applications Engineer
National Instruments