1. Tools menu -> Options -> UML -> Domain Templates -> Add Template.
2. Template Row Editor dialog appears.
3. Click on Options dialog (parent window)

The Template Row Editor dialog is hidden by the Options dialog. But there's no hint that the Options dialog is
de-activated due to the modal T.R.E. dialog.

Thus, first time around, I thought Netbeans hung, so I killed it the hard way. Not intuitive...  ;-)

For added fun, insert step 2.5 into the above sequence:

2.5. Click the Template File combo-box

When you click Options, the list section of the combo stays in front, but the T.R.E. dialog disappears!

Maybe it's a Swing thing and not Netbeans; I'm no expert in this area.