How can I type 吗 in Pinyin keyboard setup? My Mac has a "Pinyin - Traditional" keyboard mapping that shows the most frequently used Chinese characters for the sound typed (i.e. "m" or "a") so I can select the Chinese character I want. This works well for very frequently used words.

But oddly enough, even though "ma" (the question particle) is (I think) very often used, it is not in the frequent list of "m".

Is it not so often used as I think? Is there an easy way to type it using the Pinyin-Traditional keyboard mapping?
As you are using the "Pinyin - Traditional" input method, maybe what you can see will only be the traditional character "嗎". To convert it into simplified Chinese, try this tool by pasting it into the blank and click the second button.

By the way, sometimes we also use "麼" (or "么" in simplified form) at the end of a question. And the corresponding Pinyin should be "me", although most Chinese may pronounce it as "mo".