No, it can be downloaded at any time so long as you've done the extra missions in Ranger and everything.
Basically, once you finish Ranger, there are some extra missions to complete. At the end, you receive a Manaphy egg which can be sent to Pokémon Diamond. It's kind of like a trade but I think it's more like mystery gift.
HOWEVER, you can only transfer one egg from the Ranger cartridge to your other cartridge. So if you borrow a friend's copy, or you buy it used and the previous owner downloaded Manaphy already, you won't get it even if you start and finish a new game. :( I learnt that the hard way so don't make the same mistake.
It's an okay game. The whole time I was scared of scratching my touch screen with all the repetitive circles, but it turned out okay. It's not something I'd pay full price for, but if you think it looks interesting, go for it, I guess. (I might just be bitter about playing through the whole game, not liking it, and then not getting a Manaphy to make up for it. XD)