OK, it seems that Mac OS X is giving the installer incorrect information as to the location of Amnesia (and the type).
Try running these commands in the
Terminal application.
cd ~/Library/Application\ Support/Steam/SteamApps/common/amnesia\ the\ dark\ descent
touch Launcher.app
And then re-run the hpl2 tool installer.
If that still doesn't work, then you can simply "move" the tool files as they were incorrectly installed inside the Launcher application bundle in Contents/Resources folder instead.
Simply right click Launcher.app and do a show contents and move these files so they are
next to the Launcher (not inside it)
EditorSuite.plist
LevelEditor.app
MapViewer.app
MaterialEditor.app
ModelEditor.app
ModelViewer.app
ParticleEditor.app
editor
viewer