Sorry, there is no wireless support in the Slackware installer.
A way of doing it is saving the iso file on your notebook's hard disk, then mount it as a loop device see
http://slackware.com/faq/do_faq.php?faq=installation#1.
Or, you can save the entire /slackware directory of a mirror on your notebook's hard disk and tell the installer where it is.
Or, if your USB stick is big enough, put that directory on it.