There is no button for dip only.
The Free Kick system in PES is only based on how far from goal you are.
Basically, the shot button (power) changes the lenght of the balls path along a trajectory which you set using UP, DOWN, A and Y.
Combining DOWN, UP, A and Y (and also not pressing anything on the cross/stick) means a total of 9 possibilities of trajectory.
The slowest one (or highest trajectory resulting in less power) is first holding DOWN before shooting, then holding A after the shot button has been pushed and before the player shoots.
The second slowest one is only holding DOWN when shooting.
The third slowest one is holding DOWN, then pressing Y.
It goes on the same for not holding the digital cross/stick at all, and holding UP. You combine A and Y the same way.
So basically, the power bar is usually kept pretty constant. This only makes the selected trajectory get longer, and it scales fairly linearly along with the trajectories so usually there is only one power you are aiming for (one or two clicks above the centre line in the power bar, unless really far out where you need alot more power just to reach goal, or if you are really close and you need to use less power and hope the ball goes between heads in the wall to be able to get it under the bar).
The key is selecting the proper trajectory using UP, DOWN, A and Y so that the ball has enough height to go just over the wall, but still manages to dip below the bar.
According to the Official PES5 Magazine, the dip a player can put on the ball is only resulting from his Curl stat. That said, I have taken freekicks with players with 0 Curl and their kicks still manages to dip very well, only they don't get any side curl so that info seems a bit dubious to me.
Anyways, the point is that the way almost all freekick styles kick the ball, they will always give the ball topspin, which results in the ball dipping down. Results generally vary from kick to kick.
But also, it should be noted that when taking close freekicks (23-ish to 21-ish metres) holding only DOWN (or DOWN, then A for the very weakest (or highest trajectory) freekick), the ball can sometimes get a great deal of side curl, and no topspin at all. This rarely happens when shooting with lower (harder) trajectories from further out. This can sometimes lead to quite spectacular shots that curl like crazy. Of course it helps if you use a player with 99 on all stats for the highest amount of accuracy, consistency and curl.