the ESX hosts should show in the ESX tab if vOneCloud has been able to retrieve monitor information from vcenter cluster. In the hosts list the status for the host should be ON:
And if you click on the host (the one where you cannot find the ESX hosts) the state should show MONITORED or MONITORING_MONITORED:
If it shows ERROR then that would explain that ESX tab has no hosts, when in error no monitoring info is there and there's no info about esx hosts.
Could you log in to the appliance as explained here and as the oneadmin user could you run the following command: onehost show HOST_ID -x where HOST_ID should be the id of the host and paste the result?
If it shows ERROR it would also be useful to have a look to /var/log/one/oned.log from the appliance CLI or from vOneCloud logs as it may give info about a monitoring problem.