To set up your ATO using your Apex you will need a breakout box and pretty much any float switches will do the job. I got mine from a place online called Aqua Hub for like $10 a piece. I'd set up multiple float switches for redundancy. I use two switches set for low level to turn the ato pump on and one set for high level to preven sump overflows in case those two stick for whatever reason. It's also a good idea to put them inside of a specimen cup with a few holes drilled in it to prevent snails that make their way into the sump from tripping the switches. HTH
My breakout box checks 4 float switches. Low level in sump, high level in sump, low level ATO and full skimmer cup. If my water rises a few inches, my skimmer floods, so it's like a water on floor sensor and will not let the ATO flood the sump. Love being able to customize it.