You have already figured out how to launch the game, right? Then you're already halfway there.
a) Help! I don't know how to launch it!
b) This shit looks confusing!
Umm yeah it does. But we only need a select few features.
Chat: Type your message in the nifty little box and hit send. Easy enough.
Actions: That's the bold italic orange part in my screenshot. They are done by adding /me in front of the text.
eg.
/me gives Derpy a donut.
and of course the most important...
dice rolls!
/roll 1d6 will roll one 6-sided die
/roll 1d20 will roll one 20-sided die
/roll 3d100 will roll three 100-sided dice
and so on.
Go ahead and try it.