well if u dont want to cheat then u have to get i think the blue flute and go to the pokemon center and get it from some guy that is weirdly standing in front of the cash register guy and if u have like all 492 pokemon on ur pokedex. then u go to where u caught dialga/palkia and there is arceus. catch it with like a masterball.
if u want to cheat like i did and not do all that crap then get an action replay. go onto google/youtube and type in pokemon diamond または pokemon pearl cheat codes and then on the bottom of one the have a cheat 4 all the pokemon in the world. cool huh? then u can easily get arceus. hope i helped!!!!!