A search on Google for "draw Lewis structures" led to way too many unhelpful results.
The same question was asked and answeredon the Computational Chemistry list-serv from a dozen years ago: http://ftp.ccl.net/chemistry/resources/messages/1999/02/11.004-dir/index.html
Go to http://ftp.ccl.net/cgi-bin/ccl/send_ccl_message to ask your question and get an updated answer.
Theodore S. Dibble
SUNY-Environmental Science and Forestry