The styles concept in Impress is different from that implemented in Writer. There are presentation styles applying to "structural elements" of your presentation on the one hand and graphics styles applying to graphics objects on the other.
Which presentation style is applied depends on the type of the object. For example, the "outline" styles are applied to the different outline entries, and the applied style depends on the level of each single outline entry. So, to "apply" another presentation style you will have to modify the outline level, e.g. by simply increasing the level using TAB.
The same is true for the "Notes" presentation style: It will be applied automatically if you add notes using the "Notes" page of your Impress presentation.
This means: you can't simply select one of the presentation styles to modify the layout of the content. Instead, you could insert and use drawing objects using the Drawing Toolbar. For example, if you want to insert some code samples on different styles with consistent layout, you could insert text boxes using the drawing toolbar and define a Graphics Style, selecting a monospaced font and some nice background/border. That graphics style can be applied manually to different drawing objects as usual.
Of course, you can modify the presentation styles to change the style of every "outline 3" entry, for example.