I believe there isn't a custom form drop-down until you've customized the form. If you're using the standard form, I don't think it's there. Or you're using a customized form and someone has hidden the Custom Form drop-down, which is also possible. You'd have to hit Customize in the upper right-hand corner. If the form you get taken to is named "Customized Employee Record" or something like that, you were using the standard form and if you save that, the custom form drop-down should be there. If the form you get taken to is named "XYZ company Employee Record" or something that's not Customized Employee Record, then someone probably hid it. You can expose it again by checking the Show Field box next to it. I might do some asking around before exposing it to see if there's a reason someone intentionally hid it though. It's possible there is a workflow/script/integration keying off of some field that's only available on certain forms and so someone intentionally disallowed changing it.