it's not possible to apply policy to the DC and all descendant objects directly.
I would suggest:
1. Create Custom Group, use Object Type -- Virtual Machine, Filter: Relatinoship -- Descendant -- is -- <Name of your DC>. Check preview that it will returns all of your VM
2. Create a new policy as a child of your def policy, go to section 5 Selected Metric and property, click on Attribute, uncheck metric and property so only SM will be checked. Find you SM, select it and click action -- enable.
3. Go to Section 8 in the policy creation wizard and assign the policy to the CG from the step 1
4. Wait 2-3 collection cycles to check if you SM is calculated
Hope this helps