PokeWalker
In "Pokemon SoulSilver" and "HeartGold," Tropius can only be found using the PokeWalker, a small electronic pedometer that is bundled with every copy of "Pokemon SoulSilver" and "HeartGold." The PokeWalker has a small monochromatic LCD screen, and is shaped like a basic pokeball. Players can transfer one Pokemon at a time from a Nintendo DS, using the PokeWalker's infared sensor. Once you have transferred a Pokemon to the PokeWalker, you can walk around with it to take your Pokemon for a walk. For every 20 steps the PokeWalker counts, the player receives one watt.
Watts
Watts are the unit of currency in the PokeWalker, and Pokemon sometimes find bundles of 10, 20 or 50 watts when walking. Watts must be collected to unlock new areas for your Pokemon to walk in, all of which hold different species of Pokemon. Tropius can be encountered in the Big Forest, which requires 40,000 watts to access. After amassing more than 40,000 watts, walk your Pokemon through the Big Forest. After entering the forest, use the Poke Radar to look for wild Pokemon. Note that the Poke Radar costs 10 watts per use.
Poke Radar
Using the Poke Radar will bring you to a screen with four patches of grass. When the Poke Radar finds a Pokemon, between one and three exclamation marks will appear over the patch of grass where the wild Pokemon is hiding. Use the side buttons to bring the arrow over the bush with the exclamation marks before the time expires. If you select the bush fast enough, either a battle will begin, or more than one exclamation mark will appear over the same patch of grass.
Exclamation Points
In the PokeWalker, Pokemon are classified in A, B or C groups, depending on the size and power of the Pokemon. The number of exclamation points that appear above a section of grass indicate the group of Pokemon that is hiding there. Tropius is a "Group A" Pokemon, so you will need to encounter a patch of grass with two or three exclamation points -- three exclamation points corresponds to a "Group A" Pokemon, while two can mean a "Group A" or "B" Pokemon. There are two Pokemon options for each letter group, which the PokeWalker assigns at random.
Group Selection
Bonsly is the other "Group A" Pokemon option on the PokeWalker. If you encounter a Bonsly in place of a Tropius, send your Pokemon back to your DS then re-sync it with your PokeWalker, so the PokeWalker will choose Tropius as the "Group A" Pokemon in the area. If more than one exclamation point appears, move the arrow to the bush and select it again. After encountering Tropius, follow the on screen prompts to battle and capture the Pokemon, then transfer both your Pokemon and Tropius back to "SoulSilver" on your DS.