Take a US 110 V Flat Pin plug Adaptor - I bought one in ASDA a week or so ago for my upcoming trip to Playa Dorada. They will be readily available in most Supermarkets, Boots and or Electrical shops like currys or comet etc.
If you have any other questions PM me if you like regardsless of how trivial you might think it is, it's always best to be fully prepared, our up coming trip will be our fourth.