These values can be specified at the bottom of the Solids/DEM pane.
Note that the “search” feature (spyglass icon) lets you find the control for any MFiX variable. However I found that if the DEM pane was not already open, it failed to find the _YOUNG_ACTUAL and _POISSON_ACTUAL keywords, which are used as correction factors for DEM models. I will look into the reason the search failed.