The example file already has some Material settings. These were meant to paint the Meca500 model, not your new design. You will need to change these settings for your arm. Symptoms include incorrect colors or missing parts.

In the Scene tab next to each MeshInstance there is a Material node.

Change the texture. Click the texture button to see the list of loaded textures.

If your texture is not already there, click Load Texture and add it.

Select your texture and click OK.

Your robot should look like this. Notice that the DH parameters are very different from the mesh and moving the arm will look very wrong. We fix that in the next step.

Drawing DH parameters and Hinges can be turned on and off in the 3D view tab render menu.