Sounds like OmniGraffle isn't licensed - you can check by bringing up the Licenses panel via the OmniGraffle application menu.
If you have lost your license and you bought it from our online store you can get it sent to you again by going to our online store at
https://store.omnigroup.com and clicking on the "Lost Licenses" link at the bottom of the page. Enter the email address you used when you purchased OmniGraffle (or to the address we have on file, if you have had us update your email address in our records since you made your purchase), and it will look up and send your license(s) to that email address.