It sounds like you don't have '--enable-gd-native-ttf' and '--with-ttf' configured in your php, check phpinfo() to see if they are/arn't configured in.
to use TTF fonts with imagettftext() you need to first of all upload the ttf files to your server. I also noticed that it's a good idea to provide the full path of the fontfile cause for some reason it doesn't recognise relative paths very well.