You have to be in Sinnoh which is Diamond, Pearl, and Platnumn. (sorry dude or dudette.)
Pokemon Ace Trainers
Ace Trainer Joshua
you do ecruteack city
Blackthorne city is not in Pokemon Diamond but in Pokemon Heart Gold/Soul Silver
it is in the city after pewter
goldenrod city.
it is in goldrod city
in ecruteak city
By going to saffron city.
goldenrod city in a house there is a lady
Yes, you can have your Pokemon follow you in Pokemon Heart Gold, Soul Silver, and the Pokemon Ranger games. Also, certain Pokemon can follow you in Amity Square in Diamond and Pearl. (Amity Square is in Hearthome)
on the far left in the city
its in blackthorn city behind the gym
in viridian city