Travel through the listings and when you find one that you are interested in use the email contact to correspond.
Make sure to identify the position and that you found out about the position via the WIU Virtual Career Fair.
Just because they can't see you doesn't mean you can't leave a lasting impression. Make sure that your correspondence with companies is professional. When corresponding with companies make sure to attach your resumé and check your email regularly and respond to employer's requests in a timely manner.
Dear Mrs. Smith,
I am interested in the position of Sales Manager that you listed on the WIU Virtual Career Fair. I am a senior and will be graduating this December. Attached is my resume and a list of references. I look forward to hearing from you in the future.
John J. Smith
attached resume