L and R - These are the triggers, or bumpers located at the top left and top right of the DS. They aren't seen in the image above because the system is opened. In most cases cheat codes that require the use of these triggers they will be listed as L and R, and they are often the 'press and hold' type of code. This means you'll press and hold L or R (or both) while you enter another combination of buttons.
D-Pad - The D-Pad (short for directional pad) is used whenever a code requires an Up, Down, Left, or Right action. Simply use the D-Pad to enter whatever directions the code uses.
A, B, X, and Y - These are the most common buttons used for code entry on the DS. Most codes require swift yet accurate presses to work properly.
Start / Select - Not many games use Start or Select for cheat code entry on the DS, but if it call for them, I'm sure you know where they are.
Volume Up and Down - To my knowledge there are not any games that use these buttons for code entry.