Dear Patrice,
Tiled model has nothing to do with DEM, so if you are interested in DEM export, you just need to build it (Workflow menu -> Build DEM) and then export (File Menu -> Export DEM).
Tiled model is a LOD polygonal textured model. However, if there are any issues with tiled model generation, please provide the reconstruction parameters used and the Console pane output related to the procedure.