from winbox open queues on left side.
click the + sign on upper left of screen.
type in a name for this user.
BE SURE to pit the ip address in target address space or all users will be set to this speed.
set the limit in the blocks for target upload and target download.
if the speed is not listed in pull down block the type in the speed. ex 1500k is 1.5 mb
click apply and/or ok and it is set
i hope this helps