In the menu, use the mouse. In-game, you can press z, space, or click to progress the dialogue. When there are buttons for you to select to choose different options, you have to use the mouse. Credit to @silverfenn (me) for making the engine. In the main menu, the "Start Game" button asks the player for their name and starts the game, the "Voice Volume" slider sets the volume of the clicking noise that plays when characters in the game talk, the "Music Volume" slider sets the volume of the music, the "Text-to-speech" checkbox toggles text-to-speech on and off, the "Show FPS?" checkbox toggles showing the frames per second on and off, and the checkbox with the picture of the Earth toggles auto-translation into whatever the player's language is on and off. (Note: The text engine of VisScratch only supports so many characters, so if you want to make a visual novel that can translate into/is in Japanese, for example, you'll have to make the hiragana, katakana, and kanji yourself, since they're not included in VisScratch's text engine.) The "Show text all at once?" checkbox toggles showing the dialogue all at once, rather than it appearing slowly, on and off.
WARNING!! WARNING!! Contains Vocaloids And swag.