I know, its the longest title of a post.. ever. But… This is something I deal with everytime I reinstall one of my macs, and for the sake of saving some time I thought of writing it down here.
I expect you have OS X 10.8 installed (or higher), but this should also work with OS X 10.7.
Before you can use MacPorts you need to install XCode and “Command Line Tools”. You can do this by opening XCode, opening the Xcode menu and choosing “Preferences…” (pic 1). In the new window that opens, choose “Downloads” (pic 2) and select “install” next to “Command Line Tools”.