Corona Renderer Demo is a fully-functional version of Corona. It can be activated once per computer for 45 days. You can use it to try Corona out in real projects with no resolution limitations or watermarks. After 45 days, it will still open all projects, but to render them, a license purchase or downgrade to an older free build will be necessary.
To install demo, download the unified installer and install Corona. Then run 3ds Max, set renderer to Corona, and hit Render. When a licensing window pops up, select "Activate demo for 45 days". The demo will usually activate automatically. If not, you will need to copy the request key shown by Corona, paste it here, and copy the activation code back to Corona.