I used the hint for combinations that was posted on the wiki much earlier in the week. 

I'm getting an error on the line 1 <- 1 to n. 

This type of for expression was discussed in this weeks lectures, does it not work on a list of lists?

    def combinations(occurrences: Occurrences): List[Occurrences] = occurrences match {
        // for the Nil case return the empty list of lists
        case Nil => List(List())
       
        case (c, n) :: tail => {
         
        // find the combinations of the tail
          val subcombinations: List[Occurrences] = combinations(tail)
       
        //For each value i in 1 to n
        // add (c, i) to each occurrence list in subcombinations.
        // That's why addHead is of the type List[List[Occurrences]]
        // Flatten addHead and add those to subcombinations

          val addHead: List[List[Occurrences]] = for {
            i <- 1 to n
          }yield (c, i) :: subcombinations
         
            addHead.flatten
        }
    }