Your license should have been emailed to you when you bought the plugin. You can also find it when logging in at your account page: http://www.wpultimaterecipe.com/account/
You can login there with the email you used when buying the plugin and the password that was emailed to you at that time. Use the lost password page if you can’t find it anymore.
If you bought the plugin before February 2014 you’ll have to register an account first with the email address you used to buy the plugin.
Once logged in you can click through to your purchase details and find your license over there.
When WP Ultimate Recipe Premium is activated on your website you can enter your license key on top of the Recipes > Settings page: