The Inquire Cursor is the correct tool to use for determining height values of individual pixels in your DEM. If your DEM does not have elevation information defined it will not show the height value in the Elevation section under the Inquire tab, but it will still list the pixel/height value in the Inquire table below the 2D View. You can define the elevation information for your DEM using the Image Metadata tool (Image Metadata > Edit menu > Add/Change Elevation Info). Or you can temporarily assign the elevation source using the elevation section of the Status bar. The Status bar is found at the bottom of the ERDAS IMAGINE workspace and provides a constant source of information. Left-click in the elevation part of the Status bar and select Choose Elevation Source, then select your DEM and your vertical units and click OK.