Unfortunately, random vibration analyses (as well as response spectrum ones) are not supported by CalculiX. When it comes to linear dynamics analyses, only transient modal dynamic and harmonic (steady-state) vibration simulations can be performed: CalculiX: Overview of the finite element capabilities of CalculiX Version 2.18
In fact, steady-state dynamic analyses are one of the most requested features for PrePoMax. Currently, you could use keywords but it would be quite complicated. It will be somewhat easier if you apply load in form of a concentrated force acting at a reference point connected to the rest of the model with rigid body constraint.
However, it seems that the results from this type of analysis can’t be displayed in PrePoMax (likely because of their nature - they are presented in the frequency domain:
You would have to use CalculiX GraphiX to visualize such results.