I have looked into this before and here is a link I found that served my purpose I have other code if you need help not only with the list box but with all form objects. I dumped them all into a where statement when entering a form. Let me know if this helps.
dbforums.com/microsoft-access/
1677948-multi-select-list-box-into-query.html
Sorry can not post links yet. just copy paste the seperate lines into your browser.