I ran into the issue when trying to add a new host to a Nexus1K that the host selection screen was just empty. This was fairly well documented by VMware in KB2039046, even though the SQL syntax was not really.. ..perfect?! :).
For ESXi 6.x, run the following SQL query against your VC DB (replacing dvSwitch below);
DECLARE @dvs_name varchar(32); DECLARE @dvs_id int; SET @dvs_name = 'dvSwitch'; SET @dvs_id = (SELECT ID FROM VPX_ENTITY WHERE NAME = @dvs_name); INSERT INTO VPX_DVS_COMPATIBLE VALUES (@dvs_id,'esx','6.0+'), (@dvs_id,'embeddedEsx','6.0+');
Restart the vCenter service, and voilà!