If Air / Nitrox dive mode is selected, you need to define the gas(es) for the decompression algorithm to work properly. You define the gases under Gases. In Air / Nitrox mode you can change O2 percentage and PO2 value.
When you have analyzed your gas, you should round the result down when entering it for Suunto D5. For example, if the analyzed gas is 31.8% oxygen, then define the gas as 31%. This makes the decompression calculations safer.
THE DIVE COMPUTER WILL NOT ACCEPT FRACTIONAL PERCENTAGE VALUES OF OXYGEN CONCENTRATION. DO NOT ROUND UP FRACTIONAL PERCENTAGES! Rounding up will cause nitrogen percentages to be understated and will affect decompression calculations.
You can customize what you see in the Gases menu. See Customizing with Suunto DM5.