You can add any texture from the Shelf to change the material on any object added to your plan.

To change materials, go to the 3D screen. On the left your Shelf will appear. 

To use your new materials within your plan, first choose the object from your shelf you'd like to change.

To choose an object,  just touch your finger to the model and drag it to the right. Once you've dragged it, it will appear in your plan. For example, if you want to use a Cube, just search for it on your shelf under General --> Shapes. Once you've found it, drag it with your finger into your plan.

Once you've placed your object, tap on Materials at the top of your Shelf. Select the material you'd like and move your finger to the object you'd like to change. Release your finger, and the popover "Change Object Material" will appear.

Click on the icon with the material, and the material of the object will be changed.

To see an excerpt from a video tutorial on how to change materials, please visit: http://youtu.be/jv9ch5rzWxM?t=10m14s