Both of these systems are a little bit counter-intuitive. To use the radar and cloak, after you select them, there will be another icon that appears over the ship. You must Also click that one.
i.e. Step 1. Select Radar Module
Step 2. Click on the picture of the radar ping that appears.
Took me a couple turns to figure it out as well. If you're using a computer the mouseover text should say something like "activate cloak/radar ping". I'm not sure what it does on a tablet.