As far as I know, flair is a player's ability to do unexpected things (conversely, a defender's Anticipation skill is his ability to anticipate such things). Creativity determines how many options a player has in a given situation. The higher the creativity, the more options a player has to choose from.
Determination is slightly more confusing to me too. As far as I know it does a few things. First off, I believe it interacts with Aggression, Bravery, and Work Rate in deciding whether a player will actually bother beginning an action and continuing to attempt it (like going after a loose ball or something). It also decides how well or badly a player reacts to being behind in a game (or far ahead). Players with low determination will give up on matches easily as well as let up when far ahead. Lastly, determination helps decide how fast/whether a player hits his peak potential.