If you found out your Outlook signature setting is disabled or grayed out like the picture below, don’t worry. Try and do this steps.
Step 1: Open registry editor, and go to HKEY_Current_User\Software\Microsoft\Office\14.0\Common\MailSettings.
On the right, look for NewSignature and ReplySignature
Step 2: Rename NewSignature and ReplySignature to something else, such as NewSignature1 and ReplySignature1
Step 3: Refresh your registry editor and close. Now go back to your signature settings.
Now you are free to modify your signature.
If you want to disable it, just redo the steps again. 🙂