There are several alerting mechanisms in the nworks MP for Datastore usage -
Monitor for diskPercUsed - default is 95% Warn, 98% Critical
Monitor for diskFree - Default is 512MB Warn, 256MB Critical
The above is why you do not see an alert - you have not reached 95%. You can override the above thresholds in the usual SCOM way.
We also have a Monitor which triggers from the vCenter Alarm (Red, Yellow etc). This is disabled by default (because it is not usually neccessary when you have the above two thresholds).
You can see all the above Monitors if you right-click a Datastore in SCOM and go to the Health Explorer.
Hope that answers your question! Any further queries please let us know,
Vice President, Product Management