So the OSSEC installer has a conditional expression that doesn't seem to be supported with the version of Bash in Debian Squeeze.
Error:
3- Configuring the OSSEC HIDS.
./install.sh: 158: [[: not found
Fix, update line 372 on install.sh to the following:
if [ "X${USER_AGENT_SERVER_IP}" = "X" -a "X${USER_AGENT_SERVER_NAME}" = "X" ]; then
This was so frustrating, and it appears to be known issue. Unless I'm mistaken double brackets are only used in Bash to do a regular expression conditions, at least that's the only time I've used them...
UPDATE: I also ran into this same issue on FreeBSD 9 and Ubuntu Server 12.04 LTS.
Resource: https://groups.google.com/forum/?fromgroups=#!topic/ossec-list/jdl8yi5rBgI