The modeler is not included with SQL Developer. You have to download it and everything should work after that.
Actually, that should work, as Data Modeler is available as an extension to SQL Developer. Recommendations...
1. Verify that Tools > Features > Database > Oracle SQL Developer - Data Modeler is enabled.
2. It is possible you may have disabled some other feature that Modeler depends on, not sure which ones, but just be aware of possible other dependencies.
3. A Browser pane with a Designs node should appear above or below the Connections navigator pane when you View > Data Modeler > Browser
4. If it does not appear (a progress window may also indicate extensions are being loaded), try Window > Reset Windows To Factory Settings
Hope this helps.