Adding an E-junkie button to your site should just be a matter of copying the button code we provide for your product and pasting that code into your page, wherever you want the button to appear. In WordPress, you would do this in the Text tab of your page editor, which shows the raw HTML code of your page content. You may wish to type some placeholder text like XXXXXX in the Visual tab wherever you want the button to appear, so you can find that placeholder in the HTML code and replace it with your actual button code.
We are not familiar with OptimizeMember, and their online documentation for it is pretty spotty (I am guessing setup instructions are provided within your WordPress settings for OptimizeMember), so we can't tell whether E-junkie buttons will work to have members automatically added/renewed in your OptimizeMember system.
If OptimizeMember depends on manual IPN settings in your PayPal account for payment notification, E-junkie buttons send our own IPN requirements to PayPal for each order, which would override any manual IPN settings in your PayPal account; this is how our system receives payment notifications with the order data, so our system can process that order for you after payment. You could use our custom/third-party integration feature to have us forward the IPN data to your OptimizeMember IPN listener, but this may not work if that needs to receive the data directly from PayPal. We would suggest contacting OptimizeMember support with this link to inquire if they can work with this notification method: