I'm still new here so i don't know much.

To my knowledge, custom 3D objects require an OBJ file and a texture file. The 3D objects manager is at the top, and also accessible via the "Special Elements" button at the top left of the window, to the left of "File" and "Management".

After creating a new instance of a 3D object, select "Custom" from the "Shape" dropdown menu at the top, then click the button to the right of "Object:" to open the Shapes menu and choose the shape you'd like to use.

In the Shapes menu, you can open the Project and Default OBJ folders, this is where you will place the OBJ files that you want to use in your game.

Once your object is selected from the menu on the right, hit the arrow above the red X to move it into the list of usable shapes. After hitting OK, click the button to the right of the "Texture:" option to open the 2DTexture menu and select the texture for your Object.

The Textures menu also operates similar to the Shapes menu, create a new texture instance and open the Project or Default folder to select the texture that you want to use for your object.

After selecting the correlating texture with you OBJ, you should see the completed 3D object in the 3D Objects menu.

The way the whole "clicking the arrow to move the textures from the right list to the left list" feels a little bit redundant but it's very straight forward once you understand the left list is created manually based on the right list. I assume this is so that unused textures/objects/whatever don't take up space.

I modeled and textured that chair in blockbench, probably the most useful free software for voxel-based 3d modeling.