If you're seeing an invalid serial number message and assuming you're using a valid adobe serial number (6 groups of 4 numbers), you have a mismatch between your installation file and serial number. If you click a link that does not have those steps listed, open a second window using the Lightroom 3 link to see those 'Important Instructions'. They are critical especially steps 1, 2 and 3.
Downloadable installation files available:
