Please read some of the later posts. Dexcom throttles the data. Unless you have a use for driving your home automation using blood sugar readings that are 30 minutes or more old the API isn’t going to be useful for you. You would be better off setting up Nightscout and either populating it from CLARITY or using xDrip or Spike on iOS. I’ve shown how to do most of that in NightScout openHAB Integration.
And if you are so new to openHAB that you don’t know where to put Items and Rules files, I recommending stepping back and reviewing some getting started info: How to get started (there is no step-by-step tutorial).
Items go in .items files, code goes in .rules files