I hit level 5 and in the Level up description it states that's the level where you unlock the difficulty slider, etc; BUT t mssion you get that explains t difficulty slider to the player was never offered to my Trible character. Either allow the difficulty slider to be unlocked from the start; and talk about it in the main tutorial; or allow this mission to be offered at the appropriate level for players again.
OK, I'm a Lieutenant 5 and I have the mission available. It's mission that comes after doing two others in a chain.
Here's what you do: Go to or hail Quinn. Accept the mission "Medical Attention." You'll go to Spacedock sickbay and talk to the doctor, he'll give you some regenerators. He'll offer you a mission. Accept that and you'll go to the shipyard and talk to the ship repairs officer (chief engineer.) He'll give you some components and then offer you the "Difficulty Decision" mission.