Where do I find the online library to watch DVR recordings?

ATT only allows you to watch your dvr on the tv attached to it and/or tv's attached to wireless or wired att receivers in your home. 

If you want to watch and control your dvr through an internet connection you should investigate third party equipment like a Slingbox.

You can manage (not watch) your dvr using the uverse app on various mobile devices (info here).  On Demand "library" of movies and programs is available through the app as well.