Hi,
please take a look at the following threads where some capacity basics are explained
http://communities.vmware.com/thread/440552?tstart=0
http://communities.vmware.com/thread/430125?tstart=0
the "DATA STORE is Over VM Capacity by 2 VM´s because of Disk Space Usage" basically means that you have some overcommitment on the storage - probably lots of thin-disk VMs. If you feel that this situation is OK with you, and you don't need such alert, you can modify the settings accordingly.
Take a look at section 3 of the policy settings (under Configuration menu).