"Choose a player to trade with"
Here's a question: when you choose to trade with a player, at some point you have to choose a player to trade with. But you seem not able to click on anyone.
How do you choose a player to trade?
You choose after the fact. First you put out an offer to everyone with the trade button. Then you get individual responses from all the players. + means that they accept, and - means that they do not accept. Finally, you get to chose which if the players that responded with a + to actually trade with by clicking on their color coded + sign. (After this you still need to press the check mark to confirm your choice.)
The value of this is when you want to do a wild card trade. You click on the resource that you need and do not chose any resource to give. Any player that is willing to trade you that resource will respond with a +. By clicking on each + you will see what they want in return, and once chose the player with the best deal or the one you like the best, then you confirm by pressing the check mark.