If Insiders Software is still operational or has transferred its catalog to a new vendor, they usually maintain a page. You will need the email address used during purchase or the transaction ID from your credit card/PayPal statement.
Go to the app menu (usually labeled ) and select Register or Enter Serial Number . fontxchange serial number mac
If you absolutely must run FontXchange for a legacy project, consider using a virtual machine with macOS High Sierra (10.13) or earlier. But for 99% of Mac users, the best “serial number” is a credit card and a download link to RightFont, Typeface, or FontBase. If Insiders Software is still operational or has