Q:

Intellij does not recognize Scala List operator

Here is the intellij warning :

When overing "::" Intellij display this message : Cannot resolve symbol ::
I have scala + sbt plugins correctly installed.
How can i fix this error ?

A:

Your code is incorrect : :: is a method on List, not on Integer. Your last element must be an instance of List.
Either of those will work :
val otherList = 3::2::List(3)

or
val otherList = 3::2::3::Nil

Note that :: is called on List and not the Integer because it is right-associative.
From the Scala Specification (Infix Operations, 6.12.3) :

The associativity of an operator is determined by the operator’s last
  character. Operators ending in a colon ‘:’ are right-associative. All
  other operators are left-associative.

