MikuMikuDance (MMD) is a popular 3D animation software that allows users to create and customize 3D models for various projects. One common file format used for 3D models is the .obj file format. If you want to import and open .obj files in MMD, here are the steps you can follow:

1. Prepare your .obj file: Make sure you have the .obj file that you want to open in MMD. You may have created this file using a 3D modeling software like Blender, Maya, or 3ds Max.

2. Convert the .obj file to .pmd or .pmx format: MMD does not directly support .obj files, so you will need to convert your .obj file to a compatible format such as .pmd or .pmx. There are various tools and plugins available that can help you with this conversion process. Once your .obj file is converted, you can proceed to the next step.

3. Import the .pmd or .pmx file into MMD: Open MMD and go to the File menu. Select the 'Load' option and choose the .pmd or .pmx file that you have converted from the .obj file. The 3D model should now appear in the MMD viewport.

4. Adjust and customize the imported model: Once the .pmd or .pmx file is loaded into MMD, you can adjust and customize the 3D model as per your requirements. You can apply different animations, poses, and textures to the model to create your desired scene.

5. Save your project: After making all the necessary adjustments, you can save your project in MMD. This will allow you to work on the scene later or export it for use in other software or platforms.

By following these steps, you can successfully import and open .obj files in MikuMikuDance for your 3D modeling and animation projects. Keep in mind that the process may vary depending on the tools and plugins you use for file conversion. With practice and experimentation, you will become more familiar with the workflow and create impressive 3D scenes in MMD. Happy modeling!