Hello, faithful customer! You probably got here from an external site’s link, or perhaps from an old bookmark. Unfortunately, I am not currently maintaining In Series (and have not been since 2008). Nobody has requested to pick up the maintainership from me, and hence, the module has been dormant. It very likely doesn’t work with modern flavors of WordPress anymore.
I apologize for the termination of support. The WordPress development cycle in ‘08 played fast and loose with the plugin API, and churned out releases faster than I could test and verify plugin compatibility. My professional pride would not allow me to release a product without proper testing, nor would it allow me to reduce plugin compatibility (e.g., only test against WP HEAD or the newest release). The time investment required to meet both of those points of pride, however, was intractibly large; having had a day job at the time, I simply could not keep up.
The plugin and source code repository are still accessible, and licensed such that anybody can dust the plugin off and run with it. Since I no longer use WordPress, I cannot make any suggestions as to any current series-management options that would act as a feature- and quality-matched substitute.
Thank you for your interest!