1) Press and continue to hold down the [SHIFT] and [F2] buttons simultaneously. The 'USED MEMORY' window will appear on the display. The available memory is the unused portion of the 'USED MEMORY' dialog (100 - 'USED MEMORY' = Available Memory).
NOTE: The [F2] button is the second from the top of four Function buttons located to the right of the LCD panel.

Reference: Owners Manual, p. 23