I happened to have David Magie's Roman Rule in Asia Minor handy so I looked it up. You might be having problems because the name of the place changed several times in antiquity. Kabeira (also known as Cabeira) was refounded by Pompey after the defeat of Mithradates as Diospolis. In the late first century BC, Queen Pythodoris (the Roman vassal who ruled the region) renamed the city Sebaste. Finally, at some point before the end of the first century AD the city was renamed again as Neocaesareia. It is (probably) the modern day city of Niskar, and could easily be visited (though I don't think much of the ancient city survives).

The ancient geogrpaher Strabo has this to say about the site:

"Cabeira is situated close to the very foothills of the Paryadres Mountains about one hundred and fifty stadia farther south than Magnopolis, the same distance that Amaseia is farther west than Magnopolis. It was at Cabeira that the palace of Mithridates was built, and also the water-mills; and here were the zoological gardens, and, near by, the hunting grounds, and the mines." (Strabo 12.3.30)

And here, for completeness' sake, is Magie's description, which takes on board the geography around Niskar:

"Among these [villages] was Cabeira, where Mithradates Eupator had a palace and a hunting-park and where there was a 'water-mill,' presumably for grinding the grain of the neighborhood. Situated on the southern foothills of the Paryadres, the place was protected by a citadel of great strength, which rose on a projecting spur of the mountain and commanded the valley of the Lycus [River]."

I don't know what (if any) excavations have been done at the site. For a more general overview of Hellenistic palaces, you might take a look at Inge Nielsen's Hellenistic Palaces: Tradition and Renewal. This would give you at least a sense of what Mithradates' palace at Kabeira might have been like.