Mubaah is another ruling that needs to be made absolutely clear, so that people do not continue to get this one confused with some of the others. The thing that we must remember, which is most important for this particular ruling, is that there is no particular legislated request for the performance of this action. So either way, it does not really matter whether or not you carry it out.
Therefore it is the same whether or not you do it. There is no promise of either a reward for doing it, nor a punishment for its abandonment. Now, this of course is talking about the action without looking at the intention behind the action itself. For example, the eating of vegetables. If it is done for just because you like them and not for the intention of doing it to get closer to Allah by being obedient to Him, there is no reward tied to it because it is just something that is permissible to do. But if you are eating them because you are trying to stay healthy so that you can continue to worship Allah properly, then because this is done, trying to get closer to Allah, there is a promise of a reward for carrying it out.
So again, the non existence of reward is referring to actions done with out the issue of obeying Allah and gaining a closeness to Him. Whereas, the issue of seeking a reward comes not by the action itself, rather the intention to be obedient to Allah and trying to get closer to Him.