You can't get the natural frequency of a simple pendulum from a Modal analysis. The reason is that no forces are used in a Modal analysis and there is no stiffness in the simple pendulum to prevent rotation about the axis of the revolute joint.
A pendulum has a natural frequency of its swing because a gravity force acts on the mass and the string develops tension. The string has only axial stiffness, but you are not interested in the vibration of the mass axially along the length of a taut string.
You can use Rigid Dynamics if you have that in the Toolbox. It is included in the current version of the free Student license. If you don't have Rigid Dynamics, then you can use Transient Structural. Convert the sphere to a rigid body hanging from a rigid string on a revolute joint. Configure Gravity to be rotated 30 degrees about the X axis so you don't have to move the geometry to start the pendulum swinging. Make sure to make the minimum time step small enough to get 20 time steps within one period of one pendulum cycle.
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