There are two ways of hooking up a sub, basically:
1. Unamplified - this is what you see labelled as Subwoofer Output on your Yamaha.
2. Amplified. Running the main channel(s) for your front speakers into the sub first, then hooking up the speakers to the sub. This is also called speaker level.
You shouldn't need any adapters, etc. Does your sub have RCA input jacks (red and white)? If so, hook up the subwoofer out on your yamaha to either the white or red (not both).
If not, you'll end up having to use the speaker level. No adapters needed.