Being published is just icing on the cake, just like 90% of all other EC's. It obviously won't count against you if you have been published. EC's merely show that you're a well rounded, dedicated person, or at least one should strive to be that way.
In regards to being 1st, 2nd, 3rd author..etc. Being first author definately says something. The way it should be, is the person who contributes the most to the research and the paper (ie: did most of the handiwork in getting data, as well as writing the paper) should be 1st author. Second author can go either way, second author's usually would have contributed a lot but it might just be that the first author's name was there because they would attract more attention to the paper. 3rd+ authors usually are those that have provided assistance to the paper/research. This is usually where undergrad's fall into.
Of course, thats how papers should work....but I do know that there are professors out there who don't have strict standards to who gets to be what author so it may vary depending on who you're with. I'm sure adcoms know about that too, so they'd take one's publications like a grain of salt...more or less.
Other things to consider, what kind of paper was it, what journal was it published in. Or was it published in a book?
Back to the topic if being published helps or not....it may benefit you more if you're applying to an MD/PhD program. Shows that you've done well on the PhD side of things. Which would make sense since grad school likes to see if you've had good exposure to research. However at the end of the day, published or not, there are significant numbers of people who've done either/or, and have been either rejected and accepted to medical school. So do what floats your boat and be happy. Enjoy life.