To show a crosshair, simply select 'Show Crosshair' under the 'View' option in the main menu, located near the top of the SpreadsheetLIVE user interface. Once it is visible, the option will change to 'Hide Crosshair', if you decide that it's preferable not to see the crosshair. It can be seen below.
Image 1 - The 'Show Crosshair' option under the 'View' option in the top bar as well as an image of a visible crosshair.