The UP.Simulator prints status and HDML error messages to a DOS shell window entitled "Phone Information." By default, the Phone Information window is minimized, so you need to click on it in the Windows Task Manager bar to see it. If you load a deck containing invalid HDML, the UP.Phone displays a "Compile failed" message and provides further information in the Phone Information window.
For more information on UP.Simulator error and status messages, see the UP.Simulator section of the UP.SDK Tools and APIs Reference.