I'm not able to pay a credit card debt I incurred in the UK, however I
now live in the USA.  Will the credit card company be able to come
after me here?  I offered to make payments but they are insisting on
the entire amount in one payment.

Probably not! As you are now outside British jurisdiction, your UK
debts cannot be enforced against you, unless you have assets in this
country that can be seized.

A UK debt can be enforced in the USA. Once judgement has been obtained
in a British court, the normal procedure would be ask the local (USA)
equivalent of a Sheriff Officer[1] to register the certificate of
judgement with the local (USA) court. Once that has been done, the
debt can be collected under local (USA) law.

See also the comment:

Assets which you have in ... any countries outside the EU, for example
the USA, will also be part of your bankruptcy estate in the UK,
although the trustee may encounter problems in selling them if he is
not "recognised" in a foreign jurisdiction as having power to sell
them. The trustee may require co-operation from you and, also, he may
need to apply for a court order in the foreign country to deal with
the asset.
http://www.insolvency.gov.uk/freedomofinformation/technical/foreignissues.htm

[1] This is the Scottish term.

Owain

Wbar99,
Are you sure you are still being contacted by the credit card company,
or is a collection agency handling the matter now?
Because of the significant legal costs involved in pursuing the matter
in the States, they  - whichever one -  may not, but that depends on
the amount outstanding, of course.   Personally, I would be surprised
if a credit card company insisted on a single payment, but I know
nothing about UK credit card operations, nor about how collection
agencies there operate  - nor under what legal restrictions.  In the
States, these are clearly defined, as you can read by searching G-A
on:  collection agent OR agency
Perhaps it is worth the trouble to post a question asking about this. 
Or perhaps someone will volunteer some information.

Good luck, Myoarin

Hmmmmmm. Well, your UK creditor would first have to find you. AND it
would have to be convinced that you have assets that it could seize.
More likely, it would sell your debt to a collection agency for a
pittance and they would do their best but ... My guess is that you
should be OK. So let's use this as a test case and you report back
here on what happens!