Hello there.
On the dimitri you have screws on the bottom of the mod (below the position of each battery) to adjust the tightness of the contacts (they are not spring-loaded contacts). You need a screwdriver to loosen those 2 screws, install the batteries then tighten the screws till they hold the batteries snugly in place.
On the other hand if you're new to vaping I wouldn't recommend you using mechanical mods at all, until you familiarize yourself with ohms law, battery safety and other crucial information required for that application. Also note that you didn't really mention HOW new you were to these things, nor the atomizer you are using, but you will probably end up having to dive into rebuildable atomizers with mechanical mods, since that's the only way you can fine-tune your vape, since your parameters are fixed and the only way to adjust your wattage is through changing the atomizer resistance. You can find all the information you need on the forums though, so good luck and happy vaping
Cheers!