Getting the RageCandyBar
When you reach Mahogany Town, you'll find a person blocking the path to Route 44. If you speak to him, he'll offer you the RageCandyBar for $300. Accept his offer and get the item. After getting the RageCandyBar, you can leave it in your pack until later in the game when you can use it or discard it. As there is no harm in letting it sit in your inventory, there's no point in throwing it away.
Trading the RageCandyBar
Later in the game, you will find yourself in the underground path at Routes 5 and 6 near Cerulean City. Here you can trade the RageCandyBar with one of the non-playable characters for TM64. This TM contains the the Explosion move, which is one of the most powerful normal-type moves in the game. After trading the RageCandyBar, you can teach Explosion to one of your Pokemon, at which point the TM will disappear.
Differences from Generation II
"Pokemon SoulSilver" is a fourth-generation Pokemon game and a new version of "Pokemon Silver" for Game Boy Color as well as its companions "Pokemon Gold" and "Pokemon Crystal." The storyline in "SoulSilver" and many of the characters and items are updated from these games, and in some cases have been changed. In the older games, the RageCandyBar acted as a healing item that restored 20 HP to any Pokemon in your party. The item was also not a key item, which means you could acquire as many RageCandyBars as you wanted.
Trading the RageCandyBar
In "SoulSilver," you can give the RageCandyBar to one of your Pokemon and trade it to another player playing "SoulSilver" or the companion game "HeartGold," as these games have the necessary data to recognize and use the item correctly. You can also receive the RageCandyBar from a character in "HeartGold." However, because the RageCandyBar is a key item, you cannot trade with a Pokemon for a character from one of the older Generation II Pokemon games.