Baden if you could post your model thus far, and a screenshot on what inputs you are using on your iterator, that would be helpful.
Also all your raster need to be in the same folder, or if they are in subfolders, you need to select a parent folder in which they are located, and select the recursive option so it will iterate through all subfolders.
Your iterator should be at the beginning of the model, with the folder that contains the rasters being the input. the output "Raster" from that should then be the input of the DEM to Raster tool.
Hope this helps.