Q:

How do I get (a, b) => c from a => b => c in Scala? 

If I have:
val f : A => B => C

This is shorthand for:
val f : Function1[A, Function1[B, C]]

How do I get a function g with the signature:
val g : (A, B) => C = error("todo")

(i.e.)
val g : Function2[A, B, C] //or possibly
val g : Function1[(A, B), C]

in terms of f? 

A:

scala> val f : Int => Int => Int = a => b => a + b
f: (Int) => (Int) => Int = <function1>

scala> Function.uncurried(f)
res0: (Int, Int) => Int = <function2>

A:

Extending retonym's answer, for completeness
val f : Int => Int => Int = a => b => a + b
val g: (Int, Int) => Int = Function.uncurried(f)
val h: ((Int, Int)) => Int = Function.tupled(g)

The converse functions for both of these operations are also provided on the Function object, so you could write the above backwards, if you wished
val h: ((Int, Int)) => Int =  x =>(x._1 + x._2)
val g: (Int, Int) => Int = Function.untupled(h)
val f : Int => Int => Int = g.curried  //Function.curried(g) would also work, but is deprecated.  Wierd

A:

Just to round out the answer, although there is a library method to do this, it may also be instructive to do it by hand:
scala> val f = (i: Int) => ((s: String) => i*s.length)
f: (Int) => (String) => Int = <function1>

scala> val g = (i: Int, s: String) => f(i)(s)
g: (Int, String) => Int = <function2>

Or in general,
def uncurry[A,B,C](f: A=>B=>C): (A,B)=>C = {
  (a: A, b: B) => f(a)(b)
}

