Dice
Dice are only available in play mode.
Rolling

Click the dice icon on the right side of your screen to open the list of available dice.
Click as many times as needed to add dice: you can mix types in a single roll. Then click Launch to send them flying.

A cocked die will bounce on its own until it settles.
Reading the result

The result appears in the chat box at the bottom of your screen, along with the name of the player who rolled. Dice disappear on their own after a few seconds.
What exists today
Menyr offers dice from d4 to d20, with no modifiers, conditions or saved rolls.
For a roll with a bonus, do what you'd do at a physical table: roll the die, add the modifier on your sheet.
What's coming
The dice tool is being reworked alongside the interface overhaul: modifiers, conditions, roll presets. See What's coming.