Is there a Help / Sign In within LR’s menus once you start it up and get to the point where the Develop module is disabled?
If there’s Help / Sign Out instead, then do that, reboot, then Help / Sign In, again.
Thank you so much for your reply. I tried what you said and I still have the problem. I've inserted screen shots below. Since I have the membership, it seems to be that I shouldn't have to go through that first window ("serial number" or "want to try"). It seems I should be able to just login through Adobe and go straight into Lightroom. Am I doing this wrong?
Thank you for your help! Becky
Thanks for sharing the information.
You seem to have Lightroom 5 version and not CC according to your screenshot.
The installers for Lightroom 5 subscription based and standalone versions were different and you might have downloaded stand alone version which is why it is asking you for the serial number.
You need to open the Adobe Creative Cloud Application> go to Apps panel> download Lightroom CC2015 from their and then, it should be under your Applications folder by the name Adobe Lightroom.