is it under warranty? if so, don't force it in. (i did this w/ my old compaq on the PCMCIA slot and it cost me a new mobo.) if you're not under warranty u can either take it to your local apple store, compusa, or local apple certified tech. if u r not under warranty u could take it to one of the above and ask about repairs and fees, or have at it on your own.
1st off, if u try anything on your own try and get a look into the headphone jack using a flashlight, perhaps a small LED one. it's small but u should be able to see in. check to see if any of the contacts are obstructing the jack. if it is you may need to try and bend/press the offending contact to it's appropriate side of the jack. before trying this u should unplug your system and remove the battery. a utensil of choice might be a very small jeweler/tech screw driver, something that will be narrow enough to fit in and allow for some degree of visibility within the jack and also have a good amount of rigidity to it. if u can't get it to move or feel u may be messing things up worse, stop, and take it to a professional.
if your warranty is up, and u feel inspired as well as technically capable of dissasembling the notebook PM me.