thank you for your replies and suggestions!
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.
ponymad9, if this is your problem, try that, it should work! apparently it is a standard mistake newbies like me do...