Enemy Within with older DLCs

Solution 1:

XCOM: Enemy Within DOES NOT include Slingshot mission pack. You can toggle whether you want to play it in Advanced Options when starting the game ONLY if you have bought the DLC separately.

Screenshot taken in EW without Slingshot, note the presence of Meld.

When available, Operation Slingshot checkbox appears right below Operation Progeny.

Solution 2:

You can now select the various mission options including Slingshot in the "Advanced options" screen when you start a new game.

So Enemy Within includes the new missions and you have the option of playing them or not. They are available whether or not you purchased the Slingshot DLC.

When you start the game you also get asked if you want to play Enemy Within or Enemy Unknown.