Before connecting, be sure that the authorizing user has at minimum, a "moderator" role on all pages that you will be connecting to ChannelMix. The image below shows the permissions needed for "moderator" access. Alternatively, the "admin" role can be granted by toggling on the Manage Page permission.
When going through the OAuth flow, please ensure that you have "Select all" chosen or you select the pages you want ChannelMix to have access to. If you do not, you will not see the page in ChannelMix to associate a profile to.
In the case where you are reconnecting your credential, you may need to click the "Edit Settings" button after you are redirected to Facebook's sign-in page. Once you click this button, you will see the list of pages that you would like to grant access to.