"Histrionic Personality Disorder" does NOT cause a person to do the things of which Sandusky is accused.
What a person with Histrionic Personality Disorder does do, is exaggerate the emotional responses to events in life and everyday interactions.
People with this personality disorder turn situations into being about them -- no one can speak of their experiences without the person with Histrionic Personality Disorder "one upping them," so they become the center of attention, the person who has worse experiences than any other and the person to whom everyone should be paying attention. In short, they are attention seeking.
They dramatize life. They have a need to constant reassurance and approval. The emotions the individual displays are viewed by others as superficial. The person with Histrionic Personality Disorder frequently romanticizes relationships which are not romantic.
You can find more information about Histrionic Personality Disorder here
Personality disorders don't "cause" behavior, especially not sexual abuse. Personality disorders affect how a person filters information and interactions and affects their emotional response to those events.
The only thing Histrionic is Sandusky's attorneys thinking this "disorder" is a defense against what Sandusky did.
For the defense to want to use this diagnosis as an "explanation" for Sandusky's behavior tells me they are desperate - very desperate.