As many users have specified on the official forums, this procedure is not available (adding US file to UK program). To have full compatibility, you will need to install the US version on a second computer and then export or contact them directly and explain that the file won't be compatible and they have to print out all TB and supplier & Customer Balances from the US version. On the UK version, start a new data file and now input the opening balances exported from the US version.