His name is Iwan and he is one of the best guides we've ever encountered. He speaks good English, is friendly, enthusiastic and very knowledgeable about the animals in the Tangkoko reserve. Apparently, unlike most of the other guides, he works with a research project in the reserve, tracking the black macaque monkeys to observe and record their behaviour, and so he has a deep understanding of them and their habits and can easily lead tourists to find large groups of monkeys.
We visited the reserve with Iwan twice and it was one of the highlights of our 6 weeks in Indonesia. First we went in between 4-8pm and saw cuscus and tarsiers, and then we returned in the morning between 5-8am to see hornbill birds and Black Macaques (monkeys). As far as we know, we were the only tourists that evening who saw the cuscus. (Everyone sees tarsiers.) And we got to spend lots of time with 2 different groups of monkeys. There were a few other tourists when we saw the first group, but we were the only tourists visiting the second group.
If you want to contact Iwan, his details are at the website below. We were on a tour, so didn't book him directly, but he told us that he can organise transport from Manado and accommodation around Tangkoko, as well as guiding people in the reserve. Even if you arrange the other stuff yourself, we'd recommend that you book Iwan to be your guide.