You can access a source file that hasn't been mounted in Filesystems by choosing File > Open File from the main window.

In the Open dialog box, browse to and select the file you want to open. The IDE suggests a location to mount the file. You can click Accept to mount the file in the suggested location or click Select... to choose a different mount point.

After you have mounted the file, it opens in the Editor window and the file's directory is mounted in Filesystems and displayed under the Filesystems tab in the Explorer.