Hi Wouter,
This may be dependent on your model base units (which are set in Creo, not in Simulate).
I prefer to set my models to "mm N s" - this has the small disadvantage of leaving the mass units as tonnes (and density as tonnes/mm^3), but makes MPa the 'native' stress unit, and obviously N as the force unit.
If you use "mm kg s" then the force unit will be mm.kg/s^2, as you've found - the force required to accelerate 1 kg at 1 mm/s/s, otherwise known as mN. Similarly, stress will be mN/mm^2 which (if I'm right) works out to kPa.
At least Creo introduced the ability to select other units from a drop-down - in Wildfire you were stuck with the native units!
HTH!