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Often Steam tucks links to additional downloads like manuals in odd places - contact Steam customer service to find out where. In fact, when you find your serial number for Sengoku within Steam to be able to register on the forums, I'll bet you'll find the manual link right nearby.
 
I can't actually see the manual in Steam either, perhaps I am just missing it too, but if not I will check what's happening with it on Monday. In the meantime you can get it from the FAQ forum, as mentioned.