UE4 calculates mass automatically for you based on a few things:
- The simple collision geometry of the mesh
- The Density of the PhysicalMaterial applied to the object
- The MassScale parameter on the instance
You cannot specify mass directly, but can influence it through these 3 things. The reason for this is that we also need to calculate the inertia tensor of the object to make it move correctly, and we do not want you to have to type in a mass for all objects in the world, its better if we can do it automatically for you. Did you trying adjusting the MassScale? If you are seeing 0 as the mass, that probably means you do not have any simple collision on the mesh. Here are some docs on setting that up: