To add pixel for your registration page, go to your event editor, then to "Miscellaneous", then click on "Configure" for "Tracking Code Setup".
Find "Registration Page".
The first box is for placing the pixels inside <head></head>
The second box is for placing the pixels just after </head> i.e head tag closes.
The third box is for placing them just before </body> or end of body tags.
Generally pixels are placed inside head unless you are specifically told.
Once you place your pixels, don't forget to hit "Save".