you can add the servers you find on the network's webpage to your mIRC, or use the /server command.
for example, suppose the network is dudlynet (not a real network), and on their site you find a server named fred.dudly.net. in your main mIRC window to connect to that server you could type: /server fred.dudly.net
you could also click on view-->options-->connect-->server, click on the add button and add fred.dudly.net to the list for dudly.net
if you still have problems, other things could be effecting you. if so, let us know exactly what server you tried and the exact error msg