Just wondering

I downloaded all Special Missions on my Pokemon Ranger: Shadow of Almia; Liberate the Tower, Protect Manaphy's Egg, and Get Back Riolu, but how to I get the other "Extra" Missions for Dialga, Palkia, and Shaymin?

Also, what else is there to do when you beat Regigigas?

well i guess you wiat for them and for regigigas i dont know

The Extra Missions need to be downloaded from the Pokemon Ranger website. They can be found under the "Ranger Updates" section and you should find all the information you need there.