To unlock special machetes you must have an Xbox Live Silver or Gold profile.

Go to the Additional content option in the main menu and go into "Exclusive content" and follow the prompts.

When you're done linking a Ubisoft account to your Gamertag it will say "unlock special machete", click on it and you will have unlocked special machetes in the options menu.

You can only use the machetes in the single player campaign mode.