Click the "settings" icon on the bottom-left of your screen in the admin panel. Then, click the tracking tab at the top of your screen in the menu bar. After adding your default pixels, remember to click the "save" button.
You can also customize the pixel for each individual campaign. If you add a pixel on the campaign level, it will override the default pixel set for the entire account. If you remove the pixel on the campaign level, the default pixel will take back over.
To add Pixels to individual campaigns:
Click the options tab ••• to the right of the campaign in question and click the "details" button. Then, click the "Marketing button on the top-right of your page. After you've pasted your pixel(s) remember to click "save" when you are finished.
See the quick-guide below: