In Facebook roadmap documentation says:
Apps on Facebook Authentication and security migration (HTTPS)
All Canvas and Page tab apps must convert to process signed_request (fb_sig will be removed) and obtain an SSL certificate
for use in ‘Secure Canvas URL’ and ‘Secure Page Tab URL’ (unless you
are in Sandbox mode). You must provide an SSL certificate in the Dev
App settings to avoid having your app disabled.
However I have my application in sandbox mode it’s telling me to put in a secure url:
Please Update Your Secure Tab URL
To make sure users can view your app over a secure browser connection (https), please visit the developer console to update your Secure Tab URL.
I have also tried turning of secure browsing under my security settings. How can I get it to stop nagging me?
You cannot, you will have to either buy an SSL certificate, or create a self signed SLL certificate.
SSL certificates are relatively inexpensive, but if for whatever reason you need one for free during development then create a self signed certificate. Your web browser will likely display a warning, but after reassuring your browser that everything is fine, your Facebook app will work fine (for development).