I am pretty sure that you have to have FileMaker Server to use IWP. I don't think you can publish a Filemaker database to the web on any old server.
Thank you for your post.
Instant Web Publishing is definitely available under FileMaker Pro 10. If you are only seeing "FileMaker Network..." under File -> Sharing, then either this wasn't initially installed, or your account doesn't give you this permission.
Try creating a new database file with one table, one field, and one record. Pull down the File menu and select "Sharing". Does Instant Web Publishing now show? If so, then you will need to log into the old file with full read/write privileges. If Instant Web Publishing does not show, I would uninstall and do a full reinstall of FileMaker Pro 10.