Where can I find "Development / experimental stuff" window (pre-1.5)?

This guide applies to pre-1.5 versions of Corona Renderer only. To find devel/debug mode in Corona Renderer 1.5 or newer, see this updated version.

For post-A7.2 versions:

You have to enable it in render settings using "Enable devel/debug mode" checkbox located in Performance > Performance Settings. 

For A7.2 and older versions:

You can find "Development / experimental stuff" in Corona render settings after switching UI type from "Production" to "Devel/Debug". This switch is in "About Corona Renderer" rollout.