Ik heb het koud. -- I have it cold --> I am cold.
Het doet pijn. -- It does pain. --> It hurts.
Het doet mij pijn -- It does me pain. --> It hurts me.
Ik heb hulp nodig. -- I have help need. --> I need help.
Ik heb het nodig. -- I have it need. --> I need it.
Ik heb het nu nodig. -- I have it now need. --> I need it now.
I heb het onmiddelijk nodig. -- I have it immediately need. --> I need it immediately.
onmiddelijk is an adverb supporting 'to need'. Het (It) is the direct object.
If you want to say "I have immediate need." -- you could say something such as:
Ik heb een dringende noodzaak. -- "I have (an) immediate/urgent need."
There's this really nice and easy word in Dutch that means....immediately..Meteen..(see Routledge and van Dale).
I have rarely seen it in use in this program, although I have used it and heard it often.
Onmiddelijk is a much stronger word and, of course, has it's place. Like when Dad calls his naughty child' "Kom, hier, onmiddelijk!