InnoSetup, the installation tool I am using for TransDEM, silently changed the the default of one or two options in recent versions. One of those options was to pick the installation directory. And, I have to admit, I did not notice this.
To find out which installation directory the installer is looking for, open the Windows Registry Editor (regedit.exe) and look for the key
Code:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\TransDEM Trainz Edition (64bit)_is1
(assuming you are running the 64bit variant).
The registry value "InstallLocation" will show the path where the installer will apply the update.
Please note: The 32bit and the 64bit variants are separate installations.