WordPress 3.7 was dropped this past week and unfortunately it broke Organize Series in a critical way. It kind of defeats the purpose of having Organize Series active if you can’t actually add posts to a series! Yeah, that’s what happened. Anyways, I already had an update waiting in the wings to go but had a few more things I wanted to add to it before releasing. Unfortunately this bug meant releasing Organize Series a bit earlier than planned. Anyways, the fix is up on the WordPress repo and you can download it either here from the downloads page or the WordPress Plugins page.
The fix for this bug means that going forward Organize Series requires WordPress 3.7. This version of Organize Series will not work on any version of WordPress less than 3.7, so keep that in mind. Update WordPress, before you update Organize Series.
The fix also applied Organize Series Publisher as well. Everything mentioned above applies.