If trying to use an ESXi 5.x host directly that is managed by vCenter, the follow message appears and blocks that ability to proceed with the conversion:

The access to the host resource settings is restricted to the x.x.x.x that is managing the hosts. Use the management server as a destination

If the vCenter server is not available, the conversion cannot proceed.

To allow the ESXi 5.x host to be use directly as a conversion destination:

Enable SSH on the ESXi host from the console:

- Press F2 to Customise System/View Logs
- Enter root credentials
- Press F2 to Customise System/View Logs
- Select Troubleshooting Options from the menu
- Select Enable ESXi shell and press Enter

SSH to the ESXi host with root credentials and run the following commands:

/etc/init.d/hostd stop
/etc/init.d/vpxa stop
/etc/init.d/hostd start

Wait a couple of minutes for the hostd to start, then ESXi should be able to be used as a destination for converstions.

On completion of the conversions, run the following command to start the vCenter agent:

/etc/init.d/vpxa start

vCenter Converter Standalone 5.0 errors when an ESXi 5.0 host is selected as a destination: The access to the host resource settings is restricted. Use the management server as a destination (2012310)
http://kb.vmware.com/kb/2012310

Restarting the Management agents on an ESXi or ESX host (1003490)
http://kb.vmware.com/kb/1003490