Connecting to your ZNC using Colloquy is very simple. All you have to do is follow these pictures and enjoy eternal happiness.
Start by tapping "Add an New IRC Connection".
Then add the ZNC server address in the Address field, nickname being obvious (should be your IRC nickname and not your ZNC username). You can ignore the rest.
Scroll down until you see the advanced menu and tap it—here comes science. You can turn off SASL as you do not need it; enable SSL if you want, but remember to only do it if your server supports it. Add your ZNC username and password in the respective fields below and change the port if needed.