In mount coronet at the 2nd snowy part
Where Can I Catch An Absol On Platinum?
Mount cornet at the last snowy part
You first have wait patiently for Lucas' (if your a girl) or Dawn's (if your a boy) sister to tell you that Ab sol is somewhere in some Routes. I don't know which Route it is, but keep waiting until Dawn's or Lucas' sister tells you where Ab sol is found. If you were to check your National Pokedex for Ab sol, all it would say will be "Area Unknown."
In canalave city
On the top of mt. Cornet where it is snowing before you get to spear pillar
There is someone in the cave close to the pokemon leage I think you should use a master ball.
I think so you need to wait for a swam of them they are quite rare swarm's though. Good luck.