What steps do you take to install the program?
These are the typical steps on Mac:
1. Make sure you got the full download of Reason Essentials 1.5.1. It is 2 Gbytes including sound bank. There is a "update" download for users of version 1.5, that excludes the sound bank, which is identical since Essentials 1.5.
2. Uninstall the previous version of Reason Essentials from Applications by putting it in the trash using the Finder. Alternatively, rename the folder so that it is called "Reason Essentals 1.0" or something.
3. Mount the disk image or insert the DVD.
4. Drag the Reason Essentials folder from the .dmg or DVD, to Applications.
5. Start Reason Essentials from the installed copy under Applications
Note: As you start Reason Essentials for the first time, it asks you for administrator permissions to finish the installation.
PS. You can get the installation manual here: