The randomisation tools are located below the render settings on the right side of the main window.
Using them opens the Parameter Browser, which displays a number of randomly-generated thumbnails for you to choose from. This eliminates the need to re-randomise several times before obtaining a result you like.
"New Random Worlds" makes completely new paramsets, the number of which is determined by a spinbox. The palette, transforms, shaders, and selectors buttons will randomise just the indicated part of your currently loaded paramset.
Keyboard shortcuts are shown in the tooltip when you hover over the buttons.