Step 1: Create an Apple Store account. Credit cards aren't so common here because bank transfers are basicly free and you automaticly get a debit card from your bank that is universally accepted. If you try to create an account directly you will fail if you don't have a credit card. You need to "purchase" a free App to be able to create an account without credit card information.
Step 2: Download Xcode and skim the licence agreement for 10 minutes so you don't sell your organs to Apple
Step 3: Find the Console Development Tools installation somewhere hidden in the options of xcode
Step 4: Install macports
Step 5: Install octave with macports, this encompasses half an gentoo installation. Things already present on the system like the llvm from xcode get rebuild. This takes hours.
1. Installed Command Line Tools for Xcode instead of Xcode (you don't need the IDE now for build tools, and yes this used to be painful)
2. use homebrew instead of macports which is more actively maintained and generally works better