Two questions:
1. I have a copy of the contract, which lists the property address, and was signed/dated by the Tenant and the Real Estate Mgmt Firm. As the Owner, my name wasn't listed on the contract nor did I sign. Is this a valid contract?
2. I have a missing payment from my Mgmt Firm. Do I have any legal recourse to get the past due money.
Thank you!

Technically the mgmt firm might be able to claim that they don't owe you, but no judge is going to accept it as 'reasonable,' which believe it or not comes into play in the law all the time.
Before you start getting ready to sue them, what is the status of your communication with them?
And don't you have a separate agreement in writing with them, regarding things like payments? Not just copies of leases?

In response to your first question, if you have an agreement with the management company authorizing them to lease out the property on your behalf then it is not necessary for you to sign the lease for it to be valid unless the agreement specifically requires it. In response to your second question, your legal recourse is against the management company. However if you plan on filing a lawsuit then you file against both the management company and the tenant to be on the safe side. The judge will dismiss the tenant if the tenant can prove payment was made to the management company.

As Lisa said, if there is an agreement with the mgmt company to represent you, then their signature is sufficient to make the lease valid.Why are you asking?

What payment is missing? Is it a rental payment on a property they are managing for you or what?

There is a mgmt agreement, but I don't have an executed copy. A rental payment is missing/severely overdue on my house they are managing.

And.. Have you contacted them about it?

Yes! Many times (text, vm, facebook). Sadly, a friend of mine?

Have subsequent payments been made? Have you contacted the tenant to see if they have made the payment?

Yes, subsequent payments have been received and yes I have contacted the tenant. They have made their payments always on time.

And what explanation does the management company give? I would send them a registered letter telling them you will be withholding your mgmt contract payments until the missed payment is made up.

Explanation is that my "payments" got sent to someone else that he manages property for by accident. Mgmt guy was waiting for a check to be returned to him for the mistake. I think this is a coverup. Since the tenant pays the mgmt person can I "legally" have them pay me direct?

Take out your management agreement and read it through very carefully. Look for anything that refers to remedies for defaults by either party, and also look for direct contact between the landlord and tenant. If there is nothing that addresses these issues then I would send a certified letter to the tenant, with a copy sent certified to the management company, informing the tenant that all future payments should be sent directly to you. Also send copies of those letter to each of them by regular mail.

Then start shopping for a new management company.

What Lisa said. You can't just tell the tenant. You have to inform the management company as well. Plus you have to make sure that the contract does not require that payments go through them.

As for the excuse, that may very well be what happened. The thing is its not your concern. If they made a mistake and sent your payment to someone else, YOU should not have to wait until they get it back. They are responsible for paying you and for correcting their mistakes.