as long as you are not in the notes client, you only need to install on server.
if your users are opening your xpages on the notes client you need to install it on all of these machines
luckily you can push out the extlib to all your clients automatically using policys
see here for more information
http://www.wissel.net/blog/d6plinks/SHWL-8GZM9A
Good Luck
Thomas