Honestly, you’re better off using a program like Cron-o-Meter. It supports net carbs on the web and the mobile app and does a much better job. See my review of Cron-o-Meter here.

If you do still want to use MyFitnessPal:

  1. Do the install, as instructed on this page 
  2. Next you’ll want to set up your “macros” in MyFitnessPal:Go to: Home -> Goals -> Change Goals
    Select “Custom” and click continue
    I have mine set up as 70% Fat, 25% Protein and 5% Carbs.
  3. You also need to add Fiber as a column to track so the script can calculate Net Carbs for you:Go to: Food -> Settings
    Add Fiber to your list of nutrients to track. Make sure you at least have carbs, fiber, fat and protein selected. These are what you need to track for a low carb or ketogenic diet.

It ends up with a cool pie graph and net carbs on the tables (this is for a full day on Ketofood, plus a pepperjack cheese stick as a snack… and 30 minutes of intense elliptical exercise):

2014-11-24 11_53_13-Chris_bair's Food Diary _ MyFitnessPal.com