What you need for a mine is a buoy with passive sensors and a second stage missile(s), so no engines or fuel or warhead needed. A MIRV works in a similar fashion in that it deploys a secondary stage of missiles when it gets into a range (set by the thousands of km [150 in that box is 150,000km]), except with this one, because it is a missile in itself it would be a good idea to include a warhead. I don't have any missiles designed right now so I can't show exactly how they are made.

So lets start with a mine. Open your missile design window and remove everything in the design. Add a second stage missile(s) to it and set the range so it covers the range of the second stage. Add passive sensors that give a larger range than your second stage (so you can see your enemies before you fire). That's about it.

Now the MIRV. Design a missile that is mostly fuel and engines. Now add a second stage missile(s) to it and set the range so it covers the range of the second stage. The firing of those will be controlled by the ship that fired the MIRV. And that is a basic MIRV done.