You shouldn't have to put in anything. First go here (http://www.livejournal.com/voicepost/) and make sure that you've filled in all the settings.

Then pick up your phone, dial 0011 1 714 876 2711. You'll be told something like 'Welcome to the LiveJournal phonepost gateway. To log in with the number you're dialing from, press pound. To log in using another number, dial the number on your touch tone phone, and then press pound.' It'll walk you through, you do the talky bit, and then you're good to go.