Here's how to do it with the V2 version of AHK:

#Requires AutoHotkey v2.0

; Alt + I for Up Arrow
Alt & i:: {
    Send("{Up}")
}

; Alt + J for Left Arrow
Alt & j:: {
    Send("{Left}")
}

; Alt + K for Down Arrow
Alt & k:: {
    Send("{Down}")
}

; Alt + L for Right Arrow
Alt & l:: {
    Send("{Right}")
}