I haven't opened a new one in the past couple of months but ASDM 6.6(1) is compatible with ASA software 8.6(1). That was the version most units were shipping with for a while as it was the initial release that introduced support for the 5500- X series.
If the box shipped new with 9.1 ASA software then the ASDM should be at least 7.1(1) - and the recommended version is 7.2(1). Reference.
It's easy enough to upgrade ASDM - just copy the file over and change the "asdm image" command to point to it.
(By the way, you'd get better visibility of a question like this in the Security - Firewalling forum.)
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...