to use the matrix, you select the layer, the put the dimmer to full and the 3 RGB colors to full.
You can now use effects like iris, strobe etc.
To add some effects you can use gobo pages, and you will find in the custom page the setttings of the selected effect...
SOmetime to see easily the effect it's better to create a wide layout, and a layer fitting just what you need... You can this way use the matrix overview to see the effect and change the settings.