The servers ip address can be found via MSDOS (command prompt)

type ping irc.quakenet.org and it will show you the actual ip address (if it cant resolve it, you might have a DNS issue)

so typing ping irc.quakenet.org will show you the ip address is : 194.109.129.222

so type /server 194.109.129.222 in MIRC.

lastly, the reason why you couldnt connect, could be a DNS issue , this is very, very unlikely if your MSDOS can resolve the ip address of the server you wish to connect to.