Overseas/EU students who are not insured in Germany are required to submit a so-called 'Befreiungsschein' (certificate of exemption) from a German insurer. You can apply for this at any German insurance company.
The certificate of exemption can be obtained by sending a copy (front and back) of your european health insurance card (EHIC), as well as a copy of your ID/passport to the following email address email@example.com.
In a cover letter addressed to the AOK insurance company, please advise them that in order to enrol at the Freie Universität you require a 'Befreiungsschein' or a certificate confirming that you are insured abroad. Please then add a printed copy of the certificate (sent to you by the AOK) to your FU application.