Maggie,
Please contact
your American university's Study Abroad Officer . Their very job is to assist you in matters such as this.
If that fails, contact a British Embassy (you may have to telephone long distance to one the US) or phone the British Home Office (
http://www.homeoffice.gov.uk/). You may have to exit the country and then re-enter in order to obtain the appropriate work permission.
The rules regarding work permisson for foreign students in the UK changed midway through my time there. At the time I entered the UK, I did discuss my desire to work with the immigration officer. After showing that I had money to support myself without working, he said I was approved to work and stamped the passport. Curiously, the only stamp he made was one saying that I was "granted leave to remain until 30 November 2003 without recourse to public funds. Work (including [illegible smear of stamp] must be approved" That doesn't sound like work approval to me, but he said I was good to go and did not indicate that I had to register with a local JobCentre or anything.
Boy, where is UKAXO when you need her?? She used to work at the US Embassy in London.......
Anyway, definitely contact your study abroad officer to find out what steps to take next.
.....Kelly