If an apartment addendum was provided with a lease, and I have not signed/returned it to the landlord, is the lease still valid? Am I obligated to sign it? If I don't, do I lose the apartment and money?

If you were given the addeneum at the same time you were given the lease and told this addendum was part of the lease, you by signing the lease were accepting the agreement as a whole including the addendum which was most likely specific agreements to your rental, or new rules added but they did not print up a complete new lease. So in fact you have not signed the full lease for them.

If this addendum was something that you were not given with the original lease, but something added latter, no you are under no obligation to accept new terms to the lease. But can stay under the terms of the old lease till it expires.

But you did note it was provided "with" the lease, so assuming it was part of the original lease package, then there is no binding lease till all the paperwork is returned and accepted by the owner/mangement.

Actually, I was provided the addendum after I received and signed the lease. (My realtor sent it to me 3 days after the signing). I have not yet received a signed copy of my lease - this is pending upon this addendum.
How much wait time is valid (after I sign the lease, but my landlord has not, and I have not delivered this addendum) before the whole deal is cancelled?

In certain circumstances, conditions of a lease can be changed with a 30 day notice. You did not mention what was in the addendum. It would be helpful to know what the changes are. There may also be a phrase in the original lease as to any changes that may be made once the lease is signed. Please read over the original lease and also write again and tell us want changes were made in the addendum.

To answer what was in the addendum: basically reconfirming what is in the lease - in layman's terms. i.e. no pets, no painting, trash removal, etc. Nothing to disagree over.

I have not sent this back to the landlord, as I've been trying to get hold of my realtor first to discuss breaking the agreement (I thought I'd be here, but now have to move out of state). The lease would start for Sept 1.

I'm trying to work things out (and possibly not having to move out of state.) Is there a time frame from when the "unsigned" lease become invalid, or the deal is cancelled?

If I understand you correctly, the lease has been signed and in the possession of the landlord. It is only the addendum that is not signed or returned. If you signed a one year lease, then you are committed to a one year legal agreement to rent the apartment from the landlord. When you are sure of your intentions to leave, I suggest you give the landlord notice. You should consider running an ad and helping the landlord find a new tenant for the apartment. Unless there is a clause in the lease that gives you an opportunity to pay a "buy out fee" and end your legal responsibility early, you will remain obligated to pay rent for the term of the lease or until another tenant is found for that specific apartment - whichever comes first. Inform your landlord and realtor as soon as possible, in order to limit your financial obligations.