After you registered your username (not nick) with Cservice you should have recieved a password via the email address you provided. When you connect to Undernet, type /msg firstname.lastname@example.org login USERNAME PASSWORD
replacing USERNAME with the name you regisered and PASSWORD with the password you recieved.
Remember, this is not a nickname registration. You can use any nick you like, as long as it's not already being used at the time.
Next, type /mode <your nick> +x
. This will mask your IP address.