I have seen this before with Backup Exec 9.1. I had to get new license keys from Veritas/Symantec and the information store option then appeared in the selection list. It was most odd. Something to do with the upgrading of the licenses from a previous version from memory.
If they refuse to give you new keys then you will need to either
a. Buy the latest version.
b. Use NTBACKUP to do an Exchange aware backup and then backup that file.
Simon.