You can find the integrated video editor in 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!