Press 1, 2, or 3 to load a park (on the intro screen only). Select a category from the right and an item from list on the bottom. Once you have something selected, click somewhere to place it. Press E and Q to rotate, F to offset and G to variate (if possible). An eraser is in the "terrain" category. Right click the "Save" button to save or to load your park! After importing, you need to hold R to load it again. Press R and hold S to delete the park and start over.
