Web-To-Case Problem/Question

Web-To-Case Problem/Question

Hello!

We're just trying out the demoversion and are having trouble
with the web-to-case-Feature.

We would like to add more fields to the Form.
In the manual is written "click the Setup Custom Case Fields link".
But we cannot find that link anywhere... We can add custom fields to the cases themselves, but we want to add fields from e.g. the account fields to the web-form.

For example we need to realize this thing here:
The user has to enter his customer-id (which is saved along his account in the accounts) and a support-plan (also saves along his account). If he cannot supply a correct customer-id, the case/bug-report is not accepted by the system.
This prevents spam and bug-reports from non-customers or customers without a support-plan.

Is that possible?

Thanks a lot for your help!

Christian