Here’s the quick guide for how to give We Promote “manager access” (from the official Google site):
Google allows your to set up and manage a special kind of account for your business or brand, called a Brand Account. You can use this on YouTube to promote your brand or business.
To allow We Promote to run ads for you, you need to first set up a brand account. If you have created a YouTube channel, you should automatically be allocated a Brand Account.
Then you need to assign manager access to We Promote by following these steps:
0 – Login to your YouTube account.
1 – Go to your account settings: http://www.youtube.com/account
2 – Click Add or remove managers.
3 – Click Manage Permissions