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 for the file you want to open. Once you have selected a file, the Select Mount Point dialog box lets you choose where to mount it.

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