Place your mossarium in a well-lit place with natural sunlight or where it receives a couple of hours morning sun ideally.
If your mossarium gets too much sun through the glass the plants will burn and there will likely be too much condensation in the container.
Your mossarium will be working well if there is evidence of light condensation on the glass roof and walls.
If there is too much water meaning you can't see the plants clearly then remove the lid for the excess water to evaporate - this may only need to be for a couple of hours.
On the flipside, if there is no moisture around the pebbles at the bottom of the vessel or your moss is looking dry then you will need to spritz the plants with water and replace the lid.
You shouldn't need to water if in the correct position however if the top was removed by accident replace moisture with rainwater or demineralised as tap water contains chloramines which can harm your moss.