Nunu & Willump is one of those champions that make an amazing support despite of the lack of healing and crowd control. You will probably not be picking up a lot of kills with this build so you will have to rely on your team just like any other support. Nunu & Willump can also be played in the mid lane as an AP Carry but for this example we will stay in the bot lane. :)

I chose my standard Support(gold) for Nunu & Willump. If i want him to be tanky then i can buy defensive stuff from the gold that i get instead of spending my masteries on it. :)

You can switch out the runes with something else if you want to. This is just how i like to do it.

Why Exhaust
Vital spell for any support to back up a teammate in a chase. You can also go with something like Barrier to really help you defend yourself while Absolute Zero gets to its maximum damage.

Why Flash
Again this is completely personal preference but why choose Ghost when Nunu & Willump has Blood Boil?

Try rushing a philosopher's stone and a Sightstone and get some wards out there.
If you ever end up with extra gold(what???) you could go for something like Spirit Visage.