How do I open the purple door that has like 5 or 6 purple dots around it?

OK head up above the water and you should pick up a signal eventually guiding you to a room. Find the room by using a morph ball maze challenge thing. Eventually, you should find the room. Now (you need the boost ball for this) use the boost ball on the boost pads to move the rings until they eventually flashed to show that they're in place. Once this happens to all of them, the glass case in the middle will brake. This will have the seeker missiles in it. Now, when you find a purple door, press and hold Y to fire a missile and then to charge up the seeker missiles. Then (whilst keeping Y hold down) hold down R and move it over the purple power sources and once all of them are locked on to, release Y to fire a missile at all of the areas at once and destroy them all at once and then the door will worlk like a normal door for ever from then on.

This is the answer I give everyone who asks this question:
If you need the Seeker Launcher - then you must have the Super Missile and if that's the case - then you must be in the Torvus Bog right now...
So make your way back to the Transport to Torvus Bog in the Temple Grounds and go through the Green Door to the Meeting Grounds...
Here - Boost Ball up the Half Pipe and roll through the lengthy tunnel...
In the Service Access portion of the path - after the second rocky portion - you're going to need to Boost over some blocks...
Next - you'll enter the Path of Honour room - just keep going...
Now - you're in the Hall of Honoured Dead - get out of Morph Ball and you'll find yourself in a large room...
Scan the Spinner and then you're going to need to Boost in each Spinner to make the floor line up (this may take some time)...
This will remove the energy field surrounding the Seeker Launcher - now go get it...
You'll need to use it to get out of this room - it works by holding the Y-Button and using the R-Button to lock on to the 5 targets...