As of April 2, 2018, we have eliminated Multi-Tier Campaigns. The idea was originally that multiple offer groups could be associated inside a Single Campaign. But that proved too confusing to users.
Now to achieve the same thing, just create separate Campaigns for each offer you are making. Much easier! Existing Multi-Tier campaign users will see their old Campaigns have been migrated to single campaigns with the naming convention <Campaign Name> - <Tier Name>.