Opening a 3DM file can be done with various CAD software programs, the most popular being Rhino 3D. To open a 3DM file in Rhino 3D, simply launch the program and use the 'File' menu to select 'Open' and then browse to the location of the 3DM file on your computer. Once you select the file, click 'Open' to load the 3D model into the program. If you do not have Rhino 3D, there are other CAD software options that can also open 3DM files such as AutoCAD, SolidWorks, and SketchUp. Some online viewers and converters are also available for those who do not have access to CAD software. Remember that the ability to open a 3DM file may depend on the version of the software you are using, so make sure your software is compatible with the file format. With these simple steps, you can easily open and view 3DM files for 3D modeling and design purposes.