I guess you found this link as well: need OPENINFO setting sample for informix/XA access
It seems very relevant for your need. Now, I don't know anything about Informix in particular, but I guess you configure some kind of Informix environment where the dbname will resolve to a particular database instance, whether it's located on the same host or another host.
Ask your friendly Informix DBA how the database environment should be set up on your Tuxedo host. It may involve installing some software and most probably also setting environment variables before you start your Tuxedo application.
Hope this helps,