On the JavaScript part change this line
$find("AnimationExtender1").get_OnClickBehavior().play();
To
$find("FadeInAnimationBehavior").get_OnClickBehavior().play();
Add the BehaviourID attribute and set the ID to FadeInAnimationBehavior
<asp:animationextender id="AnimationExtender1" runat="server" targetcontrolid="FadeIn" xmlns:asp="#unknown">
BehaviorID="FadeInAnimationBehavior">
<animations>
<onclick>
<sequence animationtarget="PopUp">
<fadein duration="1" minimumopacity="0" maximumopacity="1" />
</sequence>
</onclick>
</animations>
</asp:animationextender>