Forward delete (Fn+delete) on short external keyboard

Hi,

I have a short (no number pad or forward delete button) external wired keyboard accompanying my macbook and want to perform a forward delete (mostly in Word and Firefox) when writing. On my macbook itself, fn+delete does this. There is an fn key on the external keyboard, but the same combination does not work, it's as if I'm not holding down fn, it's a regular backspace.

Any suggestions? How can I make the fn key modify the delete key on the external?

Thanks in advance.