You can debug only Web applications on Windows 8 in Modern UI mode; Desktop and Mobile applications are not supported.
Before you debug an application on Windows 8 in Modern UI mode, locally or remotely, you need to follow these steps:
Modify the registry setting
Modify the trusted site setting
Ensure that you've followed the steps specified in Pre-requisite to debugging an application on Windows 8 Modern UI mode.
Ensure that you've followed the steps specified in Pre-requisite to debugging an application on Windows 8 Modern UI mode for the remote computer.
If the computer on which Flash Builder is running is not on Windows 8, and you want to debug your application on Modern UI Style Internet Explorer, ensure that the project that you are debugging in Flash Builder should be a server-type project.
Alternatively, if your project is a non-server project, do the following:
Debug the application
While using Internet Explorer (IE) Modern UI browser for debugging, if you run into any issues, see this Tech Note for more information.