Looks like all acrobat.com services have been tuned out in your Reader installation which is why you do not see the Sign pane. Refer to http://helpx.adobe.com/acrobat/kb/disable-acrobat-services-acrobat-rea der.html. How have you installed Reader? Has your IT department rolled out the latest version 10.1.3? They might have blocked all acrobat.com services using the customization wizard.
Bingo! I re-set bEnableAcrobatHS and bEnableRTCPart in HKEY_CURRENT_USER\Software\Adobe\Acrobat Reader\10.0\Workflows, and now it works perfectly again.