How do unlock migrate?

You have to:
A. Beat the Pokemon league
B. Complete the pokedex(just see all the Pokemon, not catch them)

Then you go to the pal park.(to transfer Pokemon you insert your FF/LG/R/S/E game into your ds, then at the title of Pokemon D/P you go to migrate from ... And then you select the Pokemon you wish to migrate from your pc.)

Complete your sinnoh pokedex. ^-^
It's possible. You don't have to catch all the Pokemon, just see them (this doesn't include event and legendary Pokemon like heatran, manaphy, darkrai, cresselia, giratina, etc, just see all in the sinnoh dex)
There are some you will only see because trainers have them, and some that you can't catch untill later in the game, and to see palkia in diamond, an old lady has an image of palkia to show you, and this counts as one in your pokedex. The same old lady has an image of dialga in pearl version. She is in the same town as the big dialga statue/palkia statue. The grandmother you take the special item to for the young girl. (sorry I forget the town name, it's northern sinnoh right near the entry to the mountain on the east side.)

But you can complete your sinnoh dex without catching all of the Pokemon, and you only need one gamepak. Then I think either palpark just opens up, or you have to talk to someone. I think it just opens. Do you know where to find pal-park? Once it's open, you can migrate your Pokemon friends but be warned, once they move to sinnoh, they're there for keeps. They can't go back. Ever. And you can only move 6 Pokemon from each gamepak every 24 hours. Don't bother changing the time on your ds, either because when you turn your game on, the game notices, and asks you if you want to sync your gampak to the new time. If you do, it says you can't migrate any Pokemon from that gamepak (e.g. Ruby) for 24 hours!

BUT I did do this and then just switched off my ds after it said that, then tried again and it worked fine.

Have fun!

First, you need to obtain the National Dex. This is done by seeing all 150 Pokemon in the Sinnoh Dex, and THIS is easily done by simply battling all the trainers in-game to see their pokemon. And THIS is even more easier if you just beat the E4 first. Once you've seen all 150 Pokemon in your Sinnoh Dex, go to Rowan's lab in Sandgem Town and Oak will give you the National Dex. With this, go to end of Route 221 and in the building, where you'll once again meet Oak, who tells you that this is the Pal Park. He'll give you the Poke Radar which will help find some National Pokemon in the grass. [Wait, there's more! Go to his place in Eterna City and he'll give you the Up-Grade.] Save your game, and insert a GBA Pokemon game in the DS slot. Play the game [Diamond] and on the start menu, there will be an option called "Migrate from XXXX". Select that, and then 6 Pokemon from your PC in the GBA Pokemon game, that will be migrated into Diamond. Be aware that you will NEVER get these Pokemon back ever again. Then, just continue, and enter Pal Park to catch the pokemon you migrated.

First you have to beat the elite 4 and see all 50 pokemon.

Then go talk to the proffesor. You will meet proff. Oak and he will tell you to meet him at the New building south of sandjem town.

Go there and he will tell yuo about migrating.

Now you can choose migrate as a option on the main title page when a GBA game is inserted.

To unlock the migrate follow the instructions below.

1.Bate the Pokemon League
2.Complete Sinnoh pokedex

Now go to pal park by surfing from the town of the proffesor all the way to the right till you find a building which is the pal park.Save and go to the main menu at the beggining of them game click on migrate and migrate the Pokemon from your GBA Game to your DS only 6 poke.Now start the game and play pal park to catch the migrated pokemon.

First, beat the Elite 4. Insert Pokemon gameboy advanced game into the bottom of DS. Go to D/P main menu, select migrate from ___ (which is any Pokemon game). Then select up to 6 Pokemon from PC of that game. P.S Pokemon you want to migrate cannot have HMs. Then head south of Sandgem to the Pal Park.

Hope I helped