No proper guide to show how to get discount when booking through Edgars/Jet card on website

I've booked a flight for me and my son through Mango. I chose this airline since I have a Edgars Club card and the advert showed that all bookings and travel between dates 7 Aug - 30 Sep 2017 can get 25% discount. When you click on the 25% discount add, a new page opens that states the terms and conditions. When you click on the link "click here to book" it takes you back to the main page. A person will assume that you first need to make your booking and then when payment comes, the discount will be received. After phoning the helpdesk and speaking to an annoyed person, I was advised that I should first log in with my edgars account before being considered for discount. I then asked her to help me with the process since I did not get that option. She directed me to click on the right side of the page on the Edgars arrow. After clicking there, I only saw the box to enter the account number. I am not very satisfied by the structure of the website to make a booking. I've lost out now on a huge discount because the website is not properly constructed. If it had taken me to the proper box after clicking on the "click here to book" button, it would have been understandable. If it also doesn't work that way how is one to assume that it wont be deducted later since the details will be entered when making payment. I also suggest that something be put in place for people to get the discount when paying this way if there is a special again. I also sent an e-mail to the helpdesk yesterday to request clarity and no response was received.