OK, you answered my first question, you're a 2L. This makes a difference. Note, I've not gone through the process yet, but this is what I've gathered.
There is a deadline required by the NALP for when job offers can first expire. It is December 1 (that's what the Skadden folks said at a Q&A session I attended). Since it is already past that point, there probably aren't additional requirements. The firms are trying to finalize who will actually be coming over the summer so they can plan on how many 1L positions they'll have, and possibly grab any remaining 2Ls.
It is perfectly acceptable to call a firm who hasn't given you an answer yet and tell them you have a job offer and need to make a decision soon. I am presuming of course that you have already interviewed with the firm and are just waiting on a decision. You can call them (one of the interviewers probably gave you their card). Start by asking them what the status of their decision is. If they are still deciding, let them know you have an offer on the table, but need to act on it. Give them the date you need to know by.
If they want you, they'll speed things up. You can tell them that you'd really like to work at their company, but you need an offer or else you'll have to take the existing offer. If they don't want you, they won't do anything, so you'll want to more seriously consider the existing job offer.
Definitely do this through a phone call, not an e-mail. Do not leave the information in a voice message, you want to talk with someone directly to make sure someone is going to act on it. Make sure you talk with a decision-maker. You want to talk with someone who is involved in the process and can actually speed up a decision, rather than the person who was just sitting in on the interview to learn how to do a recruiting job.
I know, this is too late to help you by now, but maybe someone else will have a similar question.