Make sure you are trying to install the same version of Lightroom that you purchased i.e. 1, 2, 3, 4, 5, or 6.
If the version you purchased was an upgrade, say 4 to 5 you will have to enter the LR 5 license key first and then the earlier version. License keys are 24 numeric digits six sets of four numbers the first set are 1160.
Downloads for the earlier versions are available at the link below.