hi,
You are probably using the browser version. So basically you can use awsd or arrowkeys to navigate and to go back/cancel/exit you can use "x" and to confirm you can use "z" or "spacebar".
that was my biggest difficulty in explaining how this works. But basically it's like Triple Triad cards in Final Fantasy VIII. Maybe the resolution doesn't help anymore. At each end of the diamond there are some white dots. These white dots are the power of that corner. If it has more white dots than the opponent's corner, you will be able to convert his card to your team.
I don't know if this was your question, but if I couldn't clarify it, just say it in the comments.
