Hi Rookan,
Welcome to Civfanatics and the DuneWars mod.
You do not have to work the tile with a spice resource/harvester in order to get the boost (though they also have high tile yields).
Simply building the harvester on the spice gives a boost in commerce from your palace. Once you have harvesters built, look in your city screen to see that you have the spice resource (in the top right where resources are located) and then look at the Palace in your list of buildings and you'll see the boost.
Note that spice gives *commerce*, so you won't be getting extra gold if your science slider is still set 100%.
Hope this helps.