In order to answer this question properly, someone would need to review the original contract and the addendum. I'm guessing this is for a real estate purchase, but also knowing the subject matter of the contract would help.
Also important are the reasons that you want to back out of the contract because you might be able to get out of the contract in a different way.
However, generally, the original contract is still in effect and the modification is not in effect until ratified/signed/executed by both parties. However, if your original contract has an expiration date and the addendum has not been ratified by that date, the contract may expire.
If you really want to get out of this contract you should consult with an attorney so (s)he can review your contract and addendum and let you know where you stand.
Licensed in MD, PA and DC. This is not legal advice. I am not your attorney. You should consult with an attorney licensed to practice in your jurisdiction regarding your specific circumstances.
Please see my response to your other posting. Given the fact that you wish to get out of this contract, you need to speak with a contract lawyer to determine what your rights and options are, at this point.
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
What you describe is a classic "offer and acceptance" scenario. They offered terms of a contract in the form of an addendum, and you accepted those terms when you signed and returned the addendum. Ordinarily, you are now bound by your acceptance. The only way out is if the addendum stated it was not valid and binding until countersigned by the other party and returned to you, or some such provision. You could always try to notify them in writing that you have withdrawn your "offer" to accept the addendum since they have never counter-signed and ratified the addendum, and now consider it to be null and void. They may disagree with you, of course, and then you have potential litigation over the issue.