UIPopOver from callout MKAnnotation

I am developing an iPad app. I have a map with multiple annotations. I need to show a popup when the accessory callout method is called so that the pointer arrow points to the annotation. I am trying to use initWithRect method for popover but I am not getting correct coordinates (CGRect) in annotation. How can I get the coordinates of the annotation? I need to find out the location of this annotation in the view.