Need more info
"click on the card I want to post" what page are you on?
"it's not appearing in the box" Again what page what box?
I'm assuming you have two tabs open maybe and are adding a card to trade than going to the other page and without refreshing the page you are not seeing the card. If this is a the case then you have to refresh to page. If you change anything and want to see the change on another page you have open you have to refresh it.
You really need to describe in detail, what you're doing so we can help you.
What is that behind you!?! Oh, It's me! Looking at the cards you have, That I want. :)