You can find the integrated video editor on two different places in the tool.
It is also possible to find the video editor by going to "Library" in the left menu and then on "Materials". Further up to the right you will find a button "Add Materials". Click on the button and choose "Editable Video". Then enter the name of the video. If you click on "Add & Edit Video" you will be directed to the video editor and can start editing your new video!