please make sure to assign the custom fields to your projects. When you add/edit a custom field in the administration area, there’s a ‘Add Projects & Options’ link that opens a new dialog in which you can assign your custom field to projects and set some additional options. Once you’ve done this, the fields should appear in the case form.
Tip: I would suggest adding your custom fields to test results instead of test cases since your fields are meant to be used when executing a test as I understand it. Custom fields for test results are displayed on the following Add Result dialog (the screenshots are taken from the tour, hence the numbers):
Another alternative would be to use a test plan and configurations (for each browser you want to test against) like in the following screenshot:
Adding a test plan can be done on the Test Runs & Results tab.
Just let me know in case you have any further questions.