It is a little work to just get the agent I agree. I do not see it available form the places I can download from at least.
I would take the time to install the full VCD cell if you plan on this vCloud being used for moire than just a quick proof of concept.
Looking around
http://www.ravitech.info/2013/11/vcloud-director-55-getting-started-with.html
seems to have some information for VCD 5.5 appliance login info. (says "root/vmware" as the password)
However - I found another guide Default Username and password for VMware Virtual Appliances | VMware Arena that says "root/Default0"
Another item I have used before to change the root password of the vCenter appliance (if it expires for example)
You might be able to do something like this. (Maybe snapshot your appliance VM first in case this breaks something)
Hope that gets you in to get the agent.