In order to trek the circuit, you will need an ACAP permit. This is very easy to obtain in Kathmandu, just remember to bring 2,000 rupees (about $27 USD) and an extra passport photo to get one. All the guidebooks will tell you that you can purchase the permit upon entry to the ACAP area, and this is absolutely true... However, what they don't tell you is that they double the price of the permit if you do this. Buy it in Kathmandu. Only tourists need a permit, so don't worry about your guide.
The ACAP office in Kathmandu is located in Thamel, right under Fire and Ice Pizza. Fire and Ice Pizza is located directly across the street from Baskin Robins.