Apple Keynote’s support for LaTeX formulas is awesome: go to Insert/Equation… and enter LaTeX code directly (you can also use MathML). The code will be evaluated and results displayed as you type. Here you can find some examples. As a general rule, you should probably write multiple lines of equations as multiple individual equations and then align them using Keynote.

If however for some reason you would like to write a single multi-line equation you can use an aligned block and double backslashes to create newlines: