The recovery CD was only for older appliances like the IDS-4210, IDS-4235, and IDS-4250s.
Newer appliances and all modules do not use a Recovery CD and instead use a System Image file. The System Images have an advantage over the CD because they can often be installed remotely without requiring physical access to the sensor. (NOTE: For appliances requires remote access to the console port through a terminal server. For modules it requires remote access to the parent device.)
For the IDSM-2 you can download the 5.1 System Image file from:
http://www.cisco.com/cgi-bin/tablebuild.pl/ips5-cat6500-idsm2-sys
NOTE: IPS-IDSM2-K9-sys-1.1-a-5.1-7-E1.bin.gz is the latest system image file for the 5.1 versions.
To install the System Image file follow instructions from the CLI User's Guide:
http://www.cisco.com/en/US/partner/docs/security/ips/5.1/configuration/guide/cli/cliImage.html#wp1031426
Hopefully your upgrade to 6.x will not have any problems, and you won't need the above information.
(NOTE: Similar 6.x System Images and System Image Installation instructions also exist. )