Go to the Key Items pocket in your bag, press "Use" and you will be able to use it. For doing on the Sudowoodo blocking your way to Ecruteak City, just press "A" when your close enough to the Sudowoodo.
squirt bottle
In Pokemon Gold, the squirt bottle item can be obtained from the flower shop in Goldenrod City. It is only available to the player after the player has defeated the Goldenrod Gym.
You beat the Goldenrod City gym leader(Whitney, in SoulSilver), go to the flower shop, get the squirt bottle, go to the tree, squirt the tree, it'll attack you.
After you beat Whitney go to the flower shop and talk to the lady and she will give you a squirt bottle.
After beating the gym in Goldenrod City, go to the flower shop in the north east corner. It will be given to you there.
Talk to Whitney after you beat her. She'll be crying, so you leave and come back in a few minutes.
in Goldenrod city in a little house that's surrounded by flowers there are two girls. One of them will give you the squirt bottle, but you need to beat the gym leader first and have your first 3 badges.
Get the Squirtbottle from a lady in the town where Whitney is after beating her, and squirt the tree.
There's a house at the top right of Goldenrod City. If you have a PLAINBADGE, the woman inside will give you a SQUIRTBOTTLE.
After you beat the goldenrod Gym leader, there is a house to the right of the gym. There are 2 people inside, talk to one of them for the squirt bottle.
beat the goldenrod gym then go to the flower shop in goldenrod and she'll give you the squirt bottle
Go to the flower shop and talk to the lady there. She gives you a squirt bottle. Use it on the tree and you get a chance to catch sudowoodo.