In order to start showing notifications on your Shopify store you have at least two alternatives. You can use an app like XO Inserts Code that inserts the Pixel code for you between the <head></head> tags. Many Shopify apps are paid so you may want to find a free app that does that for you.
Another alternative involves the modification of the theme files, in particular the header.php file. You must paste the Pixel code after the opening <head> tag before the closing </head> tag.