To mark a player as having attended a training session, login to your club control panel and click Teams in the left-hand menu. After that, click Training from the expanded list of menu items.
Click into a training session listed on the page.
As seen on the training session's dedicated page, there is a section named Attendance. Underneath this are a list of players.
Next to each player use the toggle to mark them as having attended the training session. Players with a green toggle are marked as having attended the training session.