Normally you don`t need to enable IPN in your Paypal account.
You must enable it in the Payments setup page in your store for Paypal Payments Standard.
Once you enable it for the gateway ( under store -> Payments ) then WPEC will send a notify_url field to which paypal will send post data back.
If IPN is not set in your Paypal account BUT notify_url is sent to paypal with a order then the data is sent to the notify_url we send.
The below text is from the Paypal website:
Note: The IPN message will always be sent to your notification URL unless receiving IPNs have been disabled. Even though you have not enabled receiving IPN messages in your Profile or you have reset your preference by turning off IPN messages, PayPal still sends IPN messages to the notification URL you specify for a specific payment.
Read it here:
https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNSetup/#id091GJ202030