After upgrading to OS X Mountain Lion the "Mount as a Disk" feature stopped working with out any message. On top of that they dont have a support forum or database of common problems. In the end I had to search the internet to learn how to fix their software, how about a little bit of testing eh? For others who are looking if you can no longer "Mount as a Disk" try deleting ~/Library/Application Support/Transmit/Transmit Disk. Then try again.