You dont add it you replace them.
By default they are:
IK_Up=(GameKey="GI_Up",Value=1)
IK_Down=(GameKey="GI_Down",Value=1)
And should be replaced with:
IK_Up=(Gamekey=GI_AxisLeftY,Value=1.000000)
IK_Down=(Gamekey=GI_AxisLeftY,Value=-1.000000)
Left and right can be done in the Config wizard or w/e it is, but make sure to do this BEFORE changing Up + Down because the wizard will overwrite what ever changes you made to user.ini each time you load it up, so once changed don't open it up again. But I found if I deleted the following while replacing Up + Down with the above then the wizard would default to Up+Down as the movement keys instead of W+S.
IK_W=(Gamekey=GI_AxisLeftY,Value=1.000000)
IK_S=(Gamekey=GI_AxisLeftY,Value=-1.000000)