Hi Raffaello,
vCloud Director Org users can connect to the API using their vCloud Org level credentials. There are various tools that use the vCloud API, including vCloud Automation Center, vCenter Orchestrator, and PowerCLI.
VMware has a special installation of PowerCLI called "PowerCLI for vCloud Tenants" which only includes the commands that apply to vCloud Organization users (aka Tenants).
This can be downloaded from http://vmware.com/go/powercli
Let me know if you have any other questions!
Cheers,
Jake