If the data being returned is a string of numerics such as 14.3, then you should be able to use the previous answer to this question. However, if the data being returned is raw data (simply being represented as a string when returned from the VISA read function) you can type cast the data to the type you expect (in this case double). The LabVIEW type cast function is located in the following palette: All Functions -> Advanced -> Data Manipulation.
Get back if you are still having trouble!
Regards,
Jassem
Applications Engineer
National Instruments