I am going to assume that you are not using the services of a Realtor. If you are,... then you need to have him/her advise you immediately.
If not,...I would be happy to give you a few tips to work with but there just isn't enough information provided,..... starting with.... who is asking you to sign the addendum and why ?
I can tell you that addendums and extensions do happen quite often in this business but the reasons are way to many to list here...
I hope this helps a bit....
Good Luck With The Purchase...
Hank Lauzon III
Signature Real Estate Group
9525 Hillwood Drive #120
Las Vegas, NV 89134
Web : http://www.HanksLVRealestate.com
Never look down on anybody...unless you're helping them up.