1. The overlay requires DirectX Runtime to be installed. Please install DX Runtime and then restart your PC and see if the overlay works. The link below will take you to the download site for the installer: https://www.microsoft.com/en-us/download/details.aspx?id=35
2. Make sure that you have tried manually enabling the overlay with the short keys (Shift + O). These shortcut keys can be changed from settings in CAM as well.