I have a ScrollPane and this ScrollPane loads an item from the Library. And
this library item contains moviecilp and I'm trying to add 'onPress' event to
this movieclip by..

MovieName.onPress = function()
{
trace("Pressed");
}
But, since this movie is inside of ScrollPane the onPress event never occurs.

Please let me know how to an event to a movieclip that resides inside of
ScrollPane.
Thanks in advance.

The path to the clip needs to be correct.

As Luigil said, there is a path problem.
First you must find the right path.

create in level0 the var test1:String;
//Put this on Frame 1 of your loaded MC:
_level0.test1 = this; //now you can see the right path
cheers