Click on the word 'Add' in that same dialog. The IRC server is where the server address goes (i.e. irc.server.com), you can leave the port as '6667' and the group is what it will appear in the drop down list. You don't need to fill out the description if you don't want to, and unless the server is password protected (you'd know if it was or not) you don't need a password.
You don't actually *need* to add a server to that list in order to connect. If you know the server name, then simply use /server irc.server.here
and mIRC will connect to IRC. For a list of servers, try the network's website, they often have the servers listed. If you don't know the network's web address put the network's name in www.google.com
and it should come up with it.
Usually servers will have the country code in the server name *if* the server is in that area (so you can connect to a closer server geographically). For example, a European server might have .eu in it, like DALnet's powertech server is powertech.no.eu.
dal.net. If it doesn't have that, the servers are probably grouped, and you could just do /server uk.network.com (like uk.dal.net) or /server us.network.net (for an American server) and so on. If no servers match your state/country then just try the closest one (so if there's no .ca (Canada) servers, try a .us (United States) server).