Not sure I understand your question correctly but if you bought the App from Apple's App Store the license would be the Apple ID used to purchase it. If you bought the program (I use this word to differentiate the two 'versions') when it came on installable media (CD/DVD) the license would be with the media itself, either on a separate card within the box the media came in or on the cover sleeve for the media. You can also view most of the license key by launching the App, selecting Preferences from the Remote Desktop menu and from there it should be under Security? IIRC the last four digits would be asterisked out.
I use the word 'versions' to differentiate between the version you can now purchase on the App Store and the version available prior to the App Store which is now no longer officially available. This legacy version came with two distinct license keys. One for managing 10 clients and one for unlimited clients.