You go to acuity lake
Because you're not using Platinum. -hope this helps
He went to the lake near twinleaf and you have to fight team galactic
You do pretty much nothing except battle team Galactic. You do learn though that team galactic is trying to capture the Pokemon of all three lakes. This is when you travel to lake acuity.
get to lake acuity the first time and the team galactic members will stop you so then go to the snow point city gym leader and battle her and defeat her and the team galactic grunts will go away. then go inside and you will see your rival helping galactic boss Jupiter out of the water and will know the he is a fool to help Jupiter and will go away crying. after this you are allowed to go to to sunny shore city.
It is found at Lake Acuity. If you defeated it, it won't appear. Also, I think it appears after you defeat Team Galactic at Veilstone City.
After helping your rival at Lake Acuity, your next step is to complete the gym in Snowpoint City. If you've already completed the gym and have the Icicle Badge, your next step is to make it to Veilstone City to deal with Team Galactic's HQ.
In Pokemon Diamond, Jupiter doesn't go to Lake Acuity. Instead, she can be found at Team Galactic's headquarters in Veilstone City. You'll have to battle her there as part of the game's storyline.
you have to go to lake acuity near snowpoint city after defeating team galactic for good and having battled giratina. surf across the lake, enter the cave and as you speak to it you will battle it.
Team Galactic will be in Lake Valor after you win the Canalave City Gym Badge.
If you haven't defeated Maylene (the Veilstone gym leader) you need to defeat her. Then you defeat all the other gym leaders up to Candice (the Snowpoint gym leader) and have already been to Lake Acuity and have talked to Commander Jupiter of Team Galactic in Lake Acuity after you've defeated Candice. Then you go to the Team Galactic HQ, defeat Leader Cyrus and Commander Saturn of Team Glactic and press the button which will release Azelf, Uxie and Mestript from the trap. After that you have to go to Spear Pillar which is at the top of Mount Coronet. Hope this helped!
there are no team galactic grunts in that game. only team rocket, and they are not at lake of rage.