How to use Wespe or Hummel
 

Cannot seem to get them to do indirect fire.

Their fire trajectories are rather shallow on distances achievable on game maps (in reality they were used to fire at least at several kilometers distances most of the time), however, you still can use them in indirect fire role if landscape permits.
Select a unit, make sure that HE round is loaded, then click the 'attack ground' button and move mouse cursor over the map to find where it can fire. If target symbol near mouse cursor lits up, it means that the SPG can hit this spot from it's current position (LOS is not required, but aiming error is much more significant in this case). Right-click on this spot to order your SPG to fire indirectly.

thanks Sneaksie. excellent info.