I am in the US, and I tried to sign up for a Moneybookers account a few days ago. When I submitted my information, I got the following error message: "We currently do not accept customers from your country of residence." So it doesn't look like US residents are allowed to sign up for new accounts at this time. I don't know what happens if a US customer already has an account.
US Signups are currently disabled and existing USA users are not allowed to send money. Simple as that