See this: (http://processing.org/reference/mousePressed_.html)
Note the distinction with this: (http://processing.org/reference/mousePressed.html)

The first one might be called many time per frame, the other one is per frame based.

Then, you would have something like:
Code:

int arrCount = 0;
MyObject[] myArray = new MyObject[100];

class MyObject{
   MyObject(){
       //Constructor...
   }
}

void mousePressed() {
    myArray[arrCount++] = new MyObject();
} 

And, for the last part, I'm not sure if I understood correctly.