They are pointing to this thread so you can read on how to sign a message from a bitcoin address, not in order to sign a bitcoin message on this thread.

The idea is that you sign a message from an address you previously made a deposit from on the platform you are having the issue with, and post it wherever they inform you to do it (presumably, answering to their support email, but certainly not on this thread).

What exactly did they say to do? Can you copy/paste the instructions?

This is not the place to try and recover a freebitco.in account... you'll need to contact freebitco.in support. Seems like the only to do that is via a contact form hidden in the "FAQ" section on the website... after you've logged in You might have to create a new account and then contact them using that new account.

But I doubt they'll help much, given this entry on their FAQ:
A password of 12345678 is definitely not a strong password