When you have removed a VM from the vCenter Inventory, often to reinitialise the VM there is no clear way in the vCenter Infrastructure client to add it back in through the toolbar menus.
The easiest way to add a VM back in to the inventory is to locate the datastore that is hosting the VM. Select Home > Inventory > Datastores
Browse the folders and right click the corresponding .vmx configuration file and select Add to Inventory
You will be prompted for VMWare cluster and host before the VM is added back in to the inventory