On page 187 in the DS SP2 Admin guide, it shows an example of the WLogEvent command used with the -ss switch inside double-quotes. While that syntax may work for you, it didn't for me. As expected, you need the switch (-ss) outside of the quotes, and the message you are sending (if spaces are included) to be quoted.
From the manual:
.\WLogevent.exe -c:0 -l:1 "-ss:Finished with the DIR command on %NAME%"
What worked for me:
.\WLogevent.exe -c:0 -l:1 -ss:"Finished with the DIR command on %NAME%"
Another curious issue is that when attempting to use WLogEvent to return various return codes from a script, I could only get the task to process the return code correctly if the level (-l switch) was a 3; level 1 and 2 were ignored regardless of the return code provided (-c switch). Perhaps the latter was by design, but not well documented...
Perhaps everyone already knew this, but thought it was worth a FYI post.
Of course, YMMV :)