Do you mean when you click the OpenToonz app itself, or when you click a file type that you wish to be opening in OpenToonz? It sounds like you've got a MIME assignment in your OS that is associating (previously installed) ibispaint with what you now want to have associated with a newer app (i.e., OpenToonz). That sounds like an operating system environment issue, not an app level issue.
Just a guess based on what you've described above.