Welcome to the forum!
I took a look at the owner's manual, and I don't think that you've missed anything obvious. I don't see any code listings for cable boxes.
However, there is a listing in there for "Motorola". Granted, it's under the "VCR" heading, but I say give those codes a shot and see if they work for you. They may have just messed up the manual, or maybe they don't support cable boxes.
The codes are 1004 and 1008. Just something to try out.
Adam