You can enter or change your PayPal at any time in your profile.
To find your profile, open the menu by tapping the button at the top left. Tap "My Profile". Scroll down to where it has your address information. The last field is your PayPal address.
You can also enter it in the in the Payment Center - but only if your Available for Payment balance of $5 or more. If it's less, you can change it in your profile.
Please note that your PayPal email does not have to match the email you use to login to the app. Changing your PayPal email will not change your login email.