Instructions
Obtain the National Pokedex by encountering every Pokemon native to the Johto region.
Walk with your Pokewalker. Tropius is in the "Big Forest" region, which requires 40,000 watts to unlock; 40,000 watts is equal to about 800,000 steps.
Transfer one of your Pokemon over to the Pokewalker. Turn on your DS, and select "Connect to Pokewalker" on the start menu of Pokemon SoulSilver. Select "Go for a Stroll" and select one of your Pokemon. Then choose "Big Forest" as the area you want to take a stroll in. Hold your Pokewalker behind your DS and select "Connect" on it. Your Pokemon will now be transferred.
Walk around for another 6,000 steps.
Select "Pokeradar" on the Pokewalker. One of six Pokemon (one of which is Tropius) will appear. If Tropius did not appear, keep repeating the process until it does. Catch the Pokemon as you normally would in a Pokemon battle.
Transfer your Pokemon back off the Pokewalker. Select "Return from a Stroll" in the starting menu of Pokemon SoulSilver, and your two Pokemon will now be back in your game.