Hi hope this helps it does not state on this list you can not be from the USA to claim it,
you are a Commonwealth citizen
you are aged 17 or over
you have a grandparent who was born in the UK, the Channel Islands or the Isle of Man (see note below)
you have a grandparent who was born in what is now the Republic of Ireland before 31 March 1922 (see note below)
you are able to work and intend to do so in the UK, and
you can support yourself and any dependants, and live without needing any help from public funds.
Note: if you or your parent (who you are claiming ancestry through) are adopted, you can still apply for entry under this category. You must show evidence of the legal adoption with your application form.
There are other sites also.
I think Garry and Victoria would also be able to clarify all of this for you also.
Good luck