Static Meshes have both Simple Collision (the Physics Bodies that you create in your 3D art package or in the StaticMesh Editor) and Complex Collision (per poly collision) which can be composed of several different materials each with their differing Physical Materials. To set the Complex Collision Physical Material see: Assign a Physical Material to a Material.
To set the Simple Collision Physical Material for a Static Mesh:
Double-click a Static Mesh in the Content Drawer to open it in the Static Mesh Editor.
Use the Simple Collision Physical Material dropdown in the Collision category to select the desired Physical Material.
Click Save.