just so that you know how this got sorted out, in case of interest:
after much frustrations, and 2 tecchie friends trying to help me and not figuring out, i finally called Adobe Customer support! basically, what happened is that in the process of downloading LR, i signed up for the cloud (they prompt you to do so) and ended up downloading the LR cloud version, instead of the LR standard version. then of course my serial number was not working.
the tech support helped me disintall the version i had downloaded, and send me a link to the proper one.