If the mIRC Options dialog doesn't come up immediately upon opening the program, you can access it via Tools --> Options.
Put whatever you want in the "Real Name" and "Email" fields. It doesn't have to be real or working.
Put your desired nickname (WP User Name) in the "Nickname" field.
Select Servers from the menu on the left (you may have to expand "Connect" to find it)
You will probably have to Add Freenode to your list of servers. To do this:
Click Add
Type "Freenode" (without the quotes) in the description
Type "chat.freenode.net" (without the quotes) In the "IRC Server" box.
Port 6667 is fine. Use it.
You don't have to put anything in the other fields.
Click Add.
Freenode has now been added to your server list. Find it and double-click it. (It will probably be at the bottom)
This will take you back to where you put in your nickname. Click the button that says Connect to server.
The server window will scroll through connection attempts. Once you are connected, a box will pop up that says, "Enter a channel name and click join." Enter #wikimania-atlanta and click join.