are you referring on the channel trailer ?
here's some instructions how to enable it :
1. You can turn it on by clicking on the little settings icon next to your subscribe button and enable '' Customize the layout of your channel '', click save
2. You will get two tabs bellow, I assume you want to add a video for your new visitors so click on the '' For new visitors '' tab and from there you can add your video
I hope this will help you out