Using Latex symbols in Wordpress

I'm a Latex newbie. I'm using the LaTeX for WordPress and I'm having trouble implementing $$m_1\ddot{x}_1 = -k_1x_1 + k_2(x_2-x_1)$$ $$ m_1\ddot{x}_1 + k_1x_1 + k_2(x_1-x_2) = 0$$

The double dots over the x are not being recognised. What am I doing wrong?