- Go to the Google Cloud Platform Console.
 
- Click the "hamburger" menu item top left.
 
- Select Manage billing accounts.
 
- You will see a list of billing accounts and the number of projects assigned.
 
- The My projects tab will show you a list of your projects and the associated billing account.
 
To assign a billing account to a project:
- Go to IAM & admin.
 
- Select Manage resources.
 
- To the RIGHT of each project is an ellipsis menu (three vertical dots). Click and select billing.
 
If you just enabled your billing account, wait. The approval is not instant. If it has been an hour or so, go to your billing account and look for errors regarding your payment being declined, invalid credit card etc. 
Just creating a billing account does not enable a project. You must assign a billing account to a project. The menus above provide these features.
I created a videolab with a collection of videos on Google Cloud Billing. The following link is a video to show you how to create and attach a billing account. There are many more videos in this collection.
Create and attach a billing account