In order for ChannelMix to have access to your MNTN account, you'll need to connect in the Connect Page of ChannelMix Control Center.
When connecting, be prepared to provide an API Key. Here is a MNTN article on how to access API Key. The Connection Name is something you create, like "CLIENT NAME_YEAR_USERNAME".
See the image below for reference:
*NOTE: MNTN is formerly known as SteelHouse.