What credentials are you accessing the ESX servers with? Have you tried with Admin v user creds?
Also, what version of ESX are you running (3.5, 4.0, 4.1, ESXi v ESX)?
Are you using one of the free versions of ESX? If so, they restrict you to read only operations. Yu can view the state of the server and it's VMs but you are restricted from performing management operations such as restart, snapshot, and shutdown.