If not already responded to see the following:
setup (if not already) a saved putty session for the filer you wish to extract the logs from.
within the Category for the saved filer select the logging option, on the right under "session logging" select "all session output"
then within the log file name field browse to were you wish to save the log file.
Give it a name, and save this to your "saved" filer putty session"
Connect to the filer and test as you add text to the cli it will output to the log file.
Once working within the cli run the following command; rdfile /etc/messages.0
This will give you a history and output it to your log file "whereever you saved it"
hope this helps.
TA