In my experience at a college career fairs, most employers are there collecting resumes, and saying,"Apply on our website." This isn't very useful except that you can talk to someone who works there. Some companies will be actually hiring students from your school. In this case, they make make appointments for interviews to happen later that day or the next. Most often, your resume will get thrown into a pile with everyone elses'.
That is really too bad that you didn't attend in the previous years. It would have been helpful to see how they work. Be sure to dress and look professional (for males that would be slacks, shirt, tie, dress shoes, etc), even if you want a job with a company that doesn't have a dress code for developers/engineers. Lucky for you, I'm betting your college will also have a career fair in the spring. I would recommend talking to the companies that you have already selected, but don't forget other companies that are also there. I ended up getting hired into a company I'd never heard of before as a result of meeting my current manager at a college career fair my senior year.
The best way to get an in with any recruiters (preferably it's not just HR people and recruiters, but is the actual people that would be hiring/interviewing and working with you in the future-which is why it's important to make a good impression now) is to provide a demonstration of higher value while talking with them. For example, listen while they talk about the qualifications they are looking for, then explain how you meet that/have experience with that language/have seen similar scenarios in other jobs, etc. When I explained that I had used databases in a previous job, my boss put some extra stars on the resume that I handed him and put it into a different pile that all the other candidates so he would remember to follow up with me.