I recommend use YUI 2.6, get it here http://sourceforge.net/projects/yui/files/
I had troubles with YUI 2.7 and higher.
But remember if you are connected to the internet you do not need to host these files locally, just make this true:
<add key="UseGoogleCDN" value="true" />
only when you set it to false is it required to host the YUI files locally.
Hope it helps,
Joe