Unfortunately neither is really right.

Ideally the define would be used in an sprintf function call where the link would be generated using zen_href_link called to ensure not to add the session id. But if going to edit just this area of code, then it should include the applicable DIR_WS_CATALOG like constant as well. Generally speaking that would be like this:
Code:
define('EMAIL_REMINDER_REVIEW_PURCHASES_DESCRIPTION','Check out <a href="' . HTTP_SERVER . DIR_WS_CATALOG . 'index.php?main_page=addon_my_reviews">' . HTTP_SERVER . DIR_WS_CATALOG . 'index.php?main_page=addon_my_reviews</a> to find past purchases to review.');
As to landing on the account page, likely because to provide a review, the individual needs to be logged in to provide a review. Otherwise any bot could rate the product and you'd have to go through and weed things out.