Things You'll Need
Instructions
Download and install the Firebug add-on.
Open your "Pokemon Indigo" game account in the Mozilla Firefox Web browser.
Walk around in an area where there are wild Pokemon. After a while you will be prompted to engage a wild Pokemon and a "Fight" button will appear on your screen.
Click on the Firebug icon in the bottom right of your screen to start the add-on.
Scroll through the HTML source code until you find the line of text that begins "<form action>"battle.php?...". Below it are a series of input lines beginning with "<input type-"hidden" value-" and then a number. That number refers to the Pokedex number of the Pokemon that wants to fight you.
Change the Pokedex number in the source code to the Pokedex number of Dialga (483). Press "Enter" to make the changes to the source code. You will now be fighting Dialga instead of the random Pokemon you encountered.
Click "Fight" in the Pokemon Indigo game and Dialga will appear. Lower its health and use any available Ultra Balls or Master Balls to catch it just as you would any other Pokemon. You are now the proud new owner of your own Dialga.