The syslinux.cfg file is a file inside the BOOT.IMG file on the CD that provides information on how to boot up linux using the syslinux loader.
You can use the MagicISO program to decode the BOOT.IMG file and extract the syslinux.cfg file. It's not obvious how you do this though and I don't remember the commands I used to extract the files.
Syslinux.cfg is a short text file. Mine contains the following:
TIMEOUT 100
PROMPT 0
DEFAULT NASLite
LABEL NASLite
KERNEL NASLite.01
APPEND rw root=/dev/ram0 initrd=NASLite.02
DISPLAY naslite.msg
The TIMEOUT says to wait up to 100 seconds for the user to select a boot option. The PROMPT 0 says not to provide a boot prompt. The DEFAULT NASLite line says to use the information contained in the NASLite label as the default boot option. And finally the lines under the LABEL NASLite line contain information to pass to syslinux to tell it what image to boot and options to append to the boot command. You can do a web search on syslinux to learn more.
Your "unknown keyword" message means there is a word in your syslinux.cfg that syslinux doesn't know what it means.
You could also try reburning your NASLite+ CD from the ISO image. Your original CD may have become corrupted somehow.
Does your NASLite box still boot up in spite of the message? If so, I would just ignore it