To allow you to focus on the volume data display, the user interface contorls for iRay are usually hidden. You can bring them up easily at any time by choosing View -> Toggle Drawer, or pressing Command-D to show the drawer containing the controls.

Mode iRay supports two display modes, which you can select via the mode selector. The Volume mode renders the whole volume, using 3D textures for display. This mode gives the best overall impression of the 3D volume. In Slices mode, only one slice of the volume along the x, y or z axis is displayed at the time. This mode is especially suited to get a quick overview of large volume files.
Slicing When in slicing mode, a group of radio buttons allows you to choose along which axis to slice. The slider below the buttons lets you interactively choose the slice that is displayed.
Transfer Function Editor and Extended Information The two buttons in the lower part of the drawer bring up additional panels. The left button shows the transfer function editor. The right button displays extended information about the currently loaded volume file and your settings.
Brief Information Information about what volume file is currently loaded and its dimension is displayed at the bottom of the drawer. If an error occured when trying to load the volume, this is also going to be indicated here.