You will need to have account admin access on the ad accounts that you want to report on. There is a one to one relationship for Channelmix Profiles to SnapChat Ads accounts, so you may need to create extra profiles that will specify the account indicated in the connection. Often times, clients will create profiles specific to the ad account.
After making the connection and linking a profile, you will be prompted to select an Organization and Ad Account. If you do not see the Organization or Ad Account that you're looking for, then the credentials you used to create the connection do not have permissions to access that Organization or Ads accounts. You will need to get account admin access and then reconnect the connection.