The licensing mechanism changed in IJC 2.3 to reflect the new system introduced in Marvin/JChem 5. For more information about the ChemAxon licensing system see the online documentation.
To enter your Instant JChem license key choose Help -> Licenses... from the main menu bar. The standard ChemAxon license manager dialog is displayed. In this dialog you can see you current licenses and can add new licenses. The license manager allows you direct access to all the licenses that are used by IJC.
For more information about the ChemAxon license manager see the online documentation.