You can't use vCloud Director 1.5.1 or 1.5.2 with ESXi 5.1. It's just not meant to work that way.
You also cannot use ESX 4.0 or 4.1 with vCloud Director 5.1.x ... that's not going to work either.
vCloud Director 1.5.x + vSphere 4.1 or vSphere 5.0
OR
vCloud Director 5.1.x + vSphere 5.0 or vSphere 5.1
the compatibility guidelines are in the Manuals + on the web interop matrix: www.vmware.com/resources/compatibility/sim/interop_matrix.php
If this is a new installation .... start from scratch with a compatible set of software/hardware.
If this is an existing 1.5.x installation, you need to upgrade to support 5.1 esx hosts.