Connect your Pro 40 to your computer via firewire (or thunderbolt adapter if applicable). Launch Saffire Mix Control, set your sample rate and set sync source to ADAT.
Connect the ADAT output of the Octopre Mk II to the ADAT input of the Saffire Pro 40. On the Octopre Mk II make sure you set the clock source to internal / locked. Set the sample rate on the Octopre Mk II to the same sample rate as in Saffire Mix Control. Your Saffire Pro 40 should show a locked sync status and connected firewire driver.