To import an .mat file (object preset) into your object library, first enter your object library. On the 'EDIT' palette, next to the word 'EDIT,' there is a pale grey triangle pointing right. Click on this triangle. You will see the Material Library appear. Please see the screenshot below for details.
In the Material Library, you will see a preview in the upper left, one or more folders on the left and objects in various boxes in the middle and right. On the bottom, you will see the choices 'Add Delete Import Export.' Click on 'Import' and you will have a browse menu. Browse to the .mat file you wish to use, highlight, and click 'Open.' You will now see the material preset in your object library. You can also export material presets and maintain the organization of your library in this screen.
To use the material in your Bryce Scene, first select a model in your scene, then enter the Material Library and highlight the material of your choice. Click the check mark in the lower right of the Material Library. The chosen material should apply to the selected model in your Bryce scene.
A Bryce Object Preset (.mat) is *not* the same as .mat files made in other applications. A Bryce Material preset file (.mat) made in Bryce will not work in another application that may otherwise use the .mat file format. Likewise, a .mat file made in another application will not work in Bryce.