You need to provide much more info. What system are you trying to install on? Are there any error messages? Anything else we may need to know?
As Mylenium asks, please tell us more.
PrElements ships (with the physical media) with 2 separate Windows versions, 32-bit and 64-bit, and then a 32-bit Mac version. It also comes with Additional Functional Content on a separate disc, that cannot be installed, until after one has successfully installed the correct version of PrE. The markings on the discs are not really the best, and Adobe should make all that more clear.
What is your platform, Mac, or PC?
If PC, what OS, 32-bit, or 64-bit?
Exactly which disc are you trying to install from?