Sometimes its simple--sometimes not as easy as it should be. There are actually multiple ways to get to it. I'll give you step by step directions on how to do it. FatSecret sometimes has issues with updating challenges so don't panic if you get an error message. Just email them feedback and wait for them to fix it.
If you come to the challenge you'll see all of the challenge points. Scroll to the bottom of that. Between the challenge points and the forum postings, you will see a link that says "Weigh in and report". Click on that link and it will take you the weigh in page and allow you to update information for any challenges you are a part of.
You can go to the My FatSecret tab at the top of your page and under the heading "I want to:" you'll see "Weigh In." This takes you to the same screen and allows you to weigh in and report for all challenges.
I hope this helps!
“If you focus on results, you will never change. If you focus on change, you will get results.” ~Jack Dixon