You don't log into mIRC. mIRC is a chat application for an IRC network. When you connect to an IRC network, you might have to log in, especially if you're using a registered nick.
Then again, if you were using a registered nick and you were the person that registered it, then you'd know this, since you would've had to connect to the network and register the nick in the first place.
In order to provide you with a better explanation or to figure out how to help you, we need to know what's wrong, and the proper type of problem, as well as if you're getting any error messages in your status screen.