LEGO Digital Designer (LDD) is a free software developed by LEGO. It allows users to build virtual objects using virtual LEGO bricks, and it pairs great with the Virtual Robotics Toolkit.
LDraw is a popular CAD format for sharing 3D LEGO models. The simulator is capable of parsing an imported (*.ldr) file, and then assigning physical attributes to it such as weight, centre of mass, and collision data to each model element.
If you haven’t already done so, you should consider downloading and installing the LDraw "All in One" installer (https://ldraw.org/help/getting-started.html), as it will allow the simulator to work with a greater number of bricks.
After installing the "All-in-One" library, you will also want to update LEGO Digital Designer so that you can export your models to the simulation. This can be achieved by clicking on Help menu and selecting Patch LEGO Digital Designer (Help > Patch LEGO Digital Designer)
Applying the patch for LEGO Digital Designer