After installing Platinum, I haven't used it for a few months, and when I just now opened it and tried to launch Addictive drums, I just get a notice that a demo version has expired. I downloaded the xln installer, but it doesn't recognize me and wants me to register, but when I do, it says I've registered before, and shouldn't do it again.
What must I do to use the AD that came with Platinum?
 
And is Session Drummer extinct? I loved using that plugin, and made some real nice tracks with it in the past.

It's possible you need update your copy of the XLN installer. The latest version is 1.2.2 (The version number is visible on the lower right corner of the Installer GUI)

If that fails, you might want to contact XLNaudio support for the Addictive Drums issue.  Note: Addictive Drums 2 is the latest version.

https://www.xlnaudio.com/support


As regards Session Drummer being extinct, Session Drummer 3 comes with a Sonar versions except Sonar Home Studio.

Did you create an account with XLN, or just download the Demo?
Go into the main Cackewalk site, click the support menu, and choose, My Account. From their, choose 'My Products' Their is probably a redemption code or something (I don't recall as I was two years ago) that you then go to XLN site with. You then registered with XLN, and you should be fine from there

http://www.cakewalk.com/Support/Knowledge-Base/2007013382/Addictive-Drums-2-Installation-Instructions
 

Thank you, guys. I'll follow that advice..