Right clicking is the edit/move shortcut for items. If it's still not working could you please post some details such as: Your CorsixTH version, Rendering Version (SDL, OpenGL or DirectX) and perhaps a save file?

Thanks for pointing out the right click trick. It works as you stated.

No problem. It's a bit misleading that the edit button says "Edit rooms AND items"

The edit button is currently only for rooms, using it for corridor items has not been implemented

see the TO DO list