How do I get bold math in foils?

For math symbols in the running text, you can use bm{} command.

Example:

  ...$bm{y_{ij}}$ is the response...

The bm macro is different from the mathbf command in that it emboldens everything including symbols.

For math symbols in the equation or eqnarray environment, use a pair of environments:

begin{boldequation}
formula
end{boldequation}

begin{boldequation*}
formula
end{boldequation*}

The unstarred form has automatic referencing and is numbered, while the starred form has no automatic referencing and numbering.