This is a geometry node setup not an addon so no installation needed, if you want to use the node in a new Blender file just append the node (go to File>Append>look for the location of the downloaded Blender file>Node Tree>Platform Generator) or use the asset library.

It's because the node uses instances instead of real geometry to save memory. To realize the instances there is a checkbox near the top of the modifier tab. Make sure it's checked before applying the modifier, this way nothing will disappear.

Make sure to run at least Blender 4.1 as I used some nodes that were introduced in this version and there is no backward compatibility.

Make sure to have bought the full version and not the demo.

 Yes, it might not be export ready and some work is required but it should be easy enough. Also make sure to check the "realize instances" checkbox before applying the modifier. 

It might got erroneously applyied. To find it go to in the Object Data Properties tab under Attributes. There scroll until you find "UVMap" select it and click on the little down arrow to the rigth. Convert the Data type from Vector to 2D Vector and you'll get the correct UV Map.