In order to display modes of vibration, you need to construct a "modal" or eigenvalue problem. In the usual case, you assume that the field components are described by the product of a spatially dependent amplitude function and a time-dependent form like exp(i*omega*t), where omega is an angular velocity. Substitute this product for your variables and expand the resulting equations. Then select "Modes=<number>" to request that FlexPDE solve an eigenvalue problem.
You should be able to find documentation of this method in numerous places, including Backstrom's books "Simple Fields" and "Waves by Finite Element Analysis".