If this is a XFA PDF you can't edit it in Acrobat. You need a version of Livecycle Designer. You can follow the links to download a free version that will let you accomplish what you want to do.
Here are instructions on how to upgrade if you used to have an older version of Acrobat. http://helpx.adobe.com/x-productkb/policy-pricing/upgrade-livecycle-de signer-acrobat-xi.html
Adobe LiveCycle Designer Page :