well, servers do change and its hard to keep up with which are currently connected. you can check sites like www.searchirc.com
to see what they have listed for servers for the network you want to go to and join one of the servers by typing the command in your main mIRC window, like /server mesra.dal.net.
first tho, i would check to be sure you aren't being blocked from IRC. If you are at a school, lots of those do block access to IRC. Also check to be sure you arent blocking it yourself with a firewall. For example, if you are using winXP and sp2, check that firewall (see http://www.mirc.org/xpdccsp2/index.html
if you need help to do that )
other help can be found in this thread