There is a new version (20100526) of check_equallogic available for download now. I'd like to thank Roland Ripoll for his hint, that the plugin didn't work correctly on a FreeBSD system because of improper usage of the snmpwalk command. It became a bad habit to me, to set the options at the end of the snmpwalk command; the options should be set before the hostname of course (as it is even written in the very first line of the snmpwalk --help output). Shame on me!
The funny thing is though, that it seems, no Linux system didn't really care about how the snmpwalk command is being used. Another thing with FreeBSD, here the commands will not be executed correctly. Anyhow, this is now corrected and works now also on FreeBSD.