Nope no button it's a motion. From http://www.gamefaqs.com/console/wii/file/933010/46054 also http://www.gamespot.com/wii/sports/wiisports/hints.html for some hints such as how to get silver boxing gloves.
To a degree, the gloves in boxing correspond to where you actually hold the Wii Remote (right glove) and Nunchuk (left glove.) It isn't exact, but it is close -- close enough to allow you to do a Randy Orton pose or a Bruce Lee roar, if you're so inclined; but that's another story...
To do jabs, you'll have to stick either the remote or the nunchuk out, and then retract it. Do this motion with a slight angle upward to aim for the head, and at a slight angle downward to aim for the body. The faster you make this motion, then pull back from it, the more damage you'll put out with the punch. Furthermore, you can do quick 1-2s, but punching with one hand, then as soon as it finishes do a stronger punch with the second hand. If you do this correctly, a "slow-down" effect on screen will be shown, and you'll do much more damage (shown in the pie meters.)
In addition to jabs, you can also do hooks and uppercuts. Quickly move the wii remote down and to the right, at a 45 degree angle in order to do a right body hook. You can go for a right hook to the face by "turning" the remote to the left quickly, although it is a fair bit harder to do than the low hook. The same controls work for the nunchuk, to do the left-hand variations of these punches.
Uppercuts can be done by holding the wii remote or nunchuk to the ground, and quickly bringing them upward; this punch is not very trustworthy, however, and you may find yourself unable to do it when you want.
You can dodge by moving both the remote and the nunchuk to a side at the same time. Move them to the right, and you'll dodge left. Similarly, bring them both to the left, and you'll dodge to the left. You can throw punches from your leaning position, if you like. You can also block using your gloves, though. Hold them both up in front of you to block your face, and hold them both around your stomach, tilted slightly inward towards eachother to block your stomach.
If you are knocked down, rapidly jab the remote and nunchuk forward (alternating style) to help your boxer regain his feet. If he has taken a bad beating, it may not be possible, but never give up because you never know.