I did that, but there was no prompt for me to enter my serial number. Now I have lighrroom installed with my photos there, but I cannot import any photos. It simly doesn't let me. I tried from iphoto and it wouldn't work. Then I exported from iphoto onto a folder on my desktop, and tried to import those photos into Lightroom via both the Import menu and also tried to drag & drop them, no success.
I don't understand what's going on or how to fix it.
Here you'll find the full versions of Lightroom.
Download the appropriate version, deinstall your installed version and install the downloaded version.
Hope it helps.