Procedure:

Random entry:
With no text in the window above, click on "Go!" to get a random pre-analysed sentence.
Pre-analysed sentences:

Put the sentence (or part of it, or the relevant code) in the window above - or simply click on the link to the left of the sentence, if there is one.
(To view these sentences, click on "Browse the sentences" above.)

Press the Enter key or click on "Go!".

Automatic parse:

Click on "Automatic parse".
Type any text in the text field above.
Press the Enter key or click on "Go!".

These steps will start the game. More guidance can be found inside the game itself - by choosing "Help".