You type commands like that into mIRC (in the edit bar on the bottom). I gave some examples of where to find IP addresses. You should start with the network's website. For example, Undernet's website is www.undernet.org (first link in Google if you search for Undernet). On the main page, you'll see a link to their servers ( http://www.undernet.org/servers.php ) and that will show you the server names and the ports that are valid for them.

So, for example, you can type one of these:
/server Vancouver.BC.CA.Undernet.org 6667
/server bucharest.ro.eu.undernet.org 6665

Note that with the second one, I chose a port within the range that was given.

If you want the IP address, Undernet doesn't appear to list those, so you can do a lookup online. Here's one IP Address Lookup site: http://www.urgentclick.com/ip_address_lookup.php

So, if I type in bucharest.ro.eu.undernet.org that I found on Undernet's site, I get an IP address of 82.76.255.62. I can then use:

/server 82.76.255.62 6665

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