What is the Converter Script?
The easiest way to convert your scenes from other renderers is using Corona Converter script by Martin "Deadclown" Geupel. This tool is bundled within Corona installer and is installed into 3ds Max 20XX/scripts/CoronaRenderer folder. It converts most materials, maps, lights, and even proxies from Vray, Mental Ray, Final Render, and other renderers with a single click. Usually the result is very close to the original non-Corona rendering, although sometimes additional manual tweaking of materials or lights may be necessary.
How to use it?
If you cannot find the converter in 3ds Max quad menu, you will most probably need to locate the script file (scripts/CoronaRenderer/CoronaConverter_vXX.ms) and drag-and-drop it into your 3ds Max viewport. Corona Converter window should appear, with multiple conversion options and a short readme.
It is also possible to use the Converter via maxscript commands. See the full documentation with examples here: Corona Converter maxscript documentation