well I dont know if this helps
step 1.beat the elite four
2. get national dex
3. go up to mt coronet. dont go all the way
4.once you use rock climb on the fourth or fifth floor cant remember. go through this hole to your right you cant miss it.
5.go in a cave and keep going and use waterfall.
6. once you use waterfall go through a hole thats in front of you.
7. get the adamant and lustrous orb in there
8. once you you get it go to pear pillar.
9. there should be a blue or pink diamond shaped portal.
10. click on it and it should say its dialga or palkia would you like to challenge it
11. say yes.
12. once you catch one of them go back in mt. coronet.
13. then go back to spear pillar and another one should be there
14.and then catch them and you are done
i hope it helps ya ;3
and good luck with the catch