I think people are mixing up the English verb "use to" and the adjective "used to" (as in to be used to doing something). The verb "use to" indicates something that you did before but don't do anymore. It pretty much always appears in past tense, as CheBoludo says, because it indicates the past. For example: I used to walk to work but now I drive. The adjective "used to" indicates a custom or habit. It is an adjective, so it doesn't have a tense. Instead it appears with some form of the verb "be". For example, I am used to walking (I am accustomed to walking, I am in the habit of walking). I believe that the adjective "used to" should be a possible way to translate costumar in the present tense, depending on the context (costumo andar = I am used to walking, which is close in meaning to "I usually walk" though there are differences in nuance, which the context might indicate). It would be difficult for Duolingo to have a list of all the possible translations for a sentence like this since English doesn't have a single verb that would be the usual translation for "costumar" and the best translation might change from one context to another. It's a limit of a computer learning method like this. We can improve it though by suggesting answers.
I actually used, "I am used to walking.", as, one of the meanings of costumar is to be accustomed which to me is the same as being used to do something. Since Duolingo is no longer defining words I pick and choose from the dictionary to try and guess what the meaning is going to be used in the lesson.
"used to" indicates something that happened repeatedly in the past, "usually" indicates something that happens repeatedly in the present
the other usage of "used to" is not the one I believe we are referring to (someone correct me if I'm wrong), but "used to" sometimes is used to mean "accustomed to", but I don't think that's what "costumar" means, could be wrong though
It is not a verb in English though, no matter what.