First, bear in mind we have no way to handle or set up auto-recurring payment plans, so you could only use E-junkie purchase buttons if you expect buyers to make one-shot membership payments manually. If you need to have buyers set up subscriptions with a periodic (annual/quarterly/monthly/etc.) auto-recurring payment plan, you would need to use PayPal's own Recurring Payments solution by itself without involving E-junkie.
I found the WP Wishlist Member plugin's site, and from what I can tell, it works by simply redirecting buyers to a registration page after checkout, so after the buyer registers there, the plugin itself would handle emailing the buyer any registration info. If you'd be using E-junkie, this would use the product-specific version of our Redirection feature:
If you'd need some help setting things up at your end, we can recommend the competent, E-junkie-experienced developers listed in our directory here: