Its not always possible by esx permissions, that the user is allowed to browse "datacenters" of the esx cluster.
Please add the ability to add "datacenter"-name manually.

I tested it by changing the form "new compute resource" for esx, replacing datacenter dropdown with just a text field, and it works, so I was able to enter the datacenter name and then to see a list of my machines. However all other operations do not work(like create or edit machine), because then scripts want to browse for datacenters and its not permitted for my specific user.

It would be grate to have this feature!!

Regards