Moving the player up and down

In my game i want the player to have the ability to fly. Therefore, in addition to moving forward, backward, left, and right, the player should also be able to move up and down.

I checked the script of start.fps. In the file ".\client\scripts\default.bind.cs", there is such code:

function moveup(%val)
{
$mvUpAction = %val * $movementSpeed;
}

function movedown(%val)
{
$mvDownAction = %val * $movementSpeed;
}

So I add the following lines:

moveMap.bind( keyboard, t, moveup );
moveMap.bind( keyboard, g, movedown );

However, when I press "t" or "g" in the game, the player still has no action.

I searched through the engine code of player.h and player.cc, and can't found the code that controls the movement of the player. I don't know how the engine provide the player with actions like moving left or moving right, so I can't make changes to it to enable the player to move up and down.

Can someone point out which part of the code I need to make the necessary changes in order to move the player up and down? Thanks.