Make sure that you uninstall any existing version of JuiceboxBuilder-Pro first before installing a new version.
Mac: Delete the 'JuiceboxBuilder-Pro' file from the Applications folder and empty your Trash.
Windows: Use the uninstaller in the 'Control Panel -> Programs -> Uninstall a program...' list.
If you run into any problems, then the Installation Issues section of the JuiceboxBuilder-Pro User Guide may help.
If this still does not help, then please try the procedure outlined in this forum post.
Also, I would recommend that you use the latest version of Juicebox-Pro (v1.4.4.2) rather than v1.3.1 as many bugs have been fixed since v1.3.1. Please see the Version History for a list of changes between versions.
You can download the latest version using the download link from your purchase email. Full instructions can be found on the Upgrading Juicebox support page.
If you continue to experience difficulties, please let me know exactly what the problem is (what error messages are displayed, if any) and I will try to help further.