To grip the light, you use a lobster. To get the lobster, you need to go underwater and get the lost cage for Cap'n Salty.
(see the related question)
It needs to be dark
on top of the lighthouse
There is a lighthouse on Big Nate and Vampire Curse Island.
You need to get the lobster and head to the lighthouse. Use the lobster to move the light direction toward the school and the seagull. The gull will then fly away.
you dont
you use it to turn the light on the lighthouse
You will have to swim for it : the lighthouse is on Big Nate Island, not Counterfeit Island.
It is on top of the lighthouse.
aim the light from the lighthouse at the bird
The lighthouse light can be turned using a lobster to grip it. To get the lobster, dive for Cap'n Salty's lost lobster trap.(see related question)
The light house is next the next place after the playground. If you mean Big Nate Island.
It is not physically possible to go inside the Lighthouse on Big Nate in Poptroica.You don't. Everything is on the outside (comic at top, photo at bottom, light can be turned - see related question).