If you are attempting to get a QuarkXPress 5.0 serial number to work today (for legacy file retrieval), you will face modern obstacles:
If you are performing a fresh installation from original media: Insert Media quarkxpress 50 serial number work
Software retailers or Quark's authorized resellers are another option. Make sure to verify the authenticity of the reseller to avoid counterfeit products. If you are attempting to get a QuarkXPress 5