Starting point would be checking out the nursing curriculum because currently for a lot of UK trained nurses they do not meet US requirements due to the way UK training currently is. US is general trained where the UK is specialist.
Finding a school that does full time MSN may be the next issue and then it will be expensive. The school has to be approved by The Student and Exchange Visitor Program (SEVP) http://travel.state.gov/visa/temp/types/types_1268.html
Next would be immigration requirements and if you are not meeting immigration requirements you will have to move either back home or look at another country and then the process may be long