I'm guessing that the paperwork WalMart had you sign was acknowledgment of a trespass notice. Your failure to pay a civil demand will not damage your credit unless they actually sued you, won a judgment and then you failed to pay. As you've already learned from the site, that would be an unlikely outcome.
The information provided in this answer does not create an attorney-client relationship and is not considered to be legal advice.
I agree with my colleague, but I wanted to add that it would be helpful if you edited your question to clarify what exactly Wal-Mart had you sign. I presume that my colleague's guess is correct, but it would help us substantially if you clarified what you signed.
Joshua Erlich is an attorney in Arlington, Virginia. His response to your question is general in nature, as not all the facts are known to him. You should retain an experienced attorney to review all the facts in your case in order to receive advice specific to your case. Mr. Erlich's statement above does not create an attorney/client relationship.
Walmart as well as a few other retailers routinely send civil restitution demand letters to people who were caught shoplifting. The demand letters I have encountered have all been from out of state law firm that are not even located in Missouri. I have never had a client sued for not paying this and have never been told by any clients that it appeared on their credit reports.