If Final Cut Pro 5 won't open after you install it, be sure that you have QuickTime 7 installed. QuickTime 7 is required to run Final Cut Pro 5. To see what version of QuickTime you have:
Your QuickTime version number is displayed in the About window. You can download the latest version of QuickTime from here.
For more information on Final Cut Pro requirements, please see "Technical Specifications for Final Cut Pro."