As others said, it's the ablative of occiput (back of the head, occiput). Noone is ever going to learn any Latin from Alexander Lenard's absolutely terrible translation of Winnie-the-Pooh. There is nothing to salvage in this whole mess - even Lenard's grammar is shaky at best. The sentence "Ecce Eduardus Ursus scalis nunc tump-tump-tump occipite gradus pulsante post Christophorum Robinum descendens" is in fact a great example of how poor Lenard's Latin is :
"scalis" without a preposition means "with a ladder" not "in the stairs" which would be "in scalis" or "per scalas"
His thumping onomatopaea (not a good idea to use onomatopaeas in latin but - oh well) is poorly placed in the sentence.
The bear's occiput does not push or knock or slap the steps, so using pulsare is just a bad idea. I can think of several latin verbs that could have expressed the idea. Offendere and impingere come to mind.
"Descendens" : as most English speakers, and many latinists, Lenard doesn't really understand how the present participle works. He overuses it and doesn't use relative clauses enough. "Ecce ursus... descendens" sounds something like "Here is the bear... while it comes down" not "the bear coming down". In THIS context it is weird and awkward. In other contexts it would work just fine. Just as with other languages, you have to read/know enough latin to have a sense of what works in context. Of course the difficulty with latin is that there aren't so many intermediate-level texts to get to that level in the first place. Lenard and his Winnie-ille-Pu are just a fraud.
I read a few pages of it on the web and was horrified. It is a sloppy, lazy and faulty translation, by someone who didn't have good enough command of the language (let alone of its stylistics) to have any business translating a book, or anything, into latin. Even Harrius Potter is better, and it's pretty damn bad.