Keynote & Pages formular editor - LaTex: Umlaut ä could be forced with \\ddot{a}.

As Grant Bennet-Alder suggested, you can press and hold the letter 'a' to select the umlauted character whether external to the equation editor, or within it. No need to use LaTeX \ddot{a}.

\text{ä}

The \ddot{a} does produce a smaller italicized ä than the preceding roman text version above.

The following is all that Apple provides, assuming one is already versed in LaTeX. The link to blatex in the following article is the PDF guide to the LaTeX support in the Equation editor.

About LaTeX and MathML support in Pages, Numbers and Keynote - Apple Support (IN)

To ask Apple to improve its forumula editor, you can use

http://www.apple.com/feedback

ä Â

those characters are not so special. They can be typed directly form the Mac keyboard, and do not require convoluted LaTex strings.

Enter characters with accent marks on Mac - Apple Support