I checked again, there was a problem with copying over the bundled extensions. Fixed in build 8964.
To clarify because what I said above was not quite correct. When you install Stencyl, it copies the plaf/haxe/extensions contents to your workspace/engine-extensions. If you don't have them there they don't show up. You can delete or modify the extensions in your workspace and you can always restore the bundled ones by copying them again from the install dir to your workspace.