In the description there is this:
Drop the tablet in different spots of the aquarium, for more even distribution of CO2
2 tablets for every 10 gallon of water, dose 2 times a week
So seems like it's ok to just drop them in your tank.
But I've also already seen kits like this where a small basket was included with suction cups, where you could put the tablet in. This might be safer, so your fish won't confuse it with any food tablets.
Also, don't worry about killing your plants. But do keep an eye out for your fishes. High CO2 means less oxygen in your tank. So if you see your fish gasping for air, you should reduce the dose you add.
On a related note, you can also buy liquid CO2. In my nano tank I use EasyLife Carbo. Very good product, and I think in the end it will be cheaper than those tables. Flourish Excel is another product (but I don't have experience with this).
You can also try a DIY setup. I'm using that for my big tank (180 liters) and it works very good. I followed this guide: http://www.hobbykwekers.nl/artikelen/item/co2-verhogen-met-omgekeerd-bakje - it is in Dutch, but pictures are clear and you can use google translate. And if you google it, I'm sure you'll find similar guides in English. That reversed container makes sure you don't get too much CO2 in your tank (that container surface should be roughly 10 % of your tank surface).
For my nano tank I found it difficult to get a decent CO2 production rate. It seemed like a lot of the CO2 went to waste because of the small container.
But if you play around with the mixture you might find something that works for you (less sugar and yeast).