An instrument driver for the 34401 has shipped with LabVIEW for a very long time. Go to your Instrument I/O>Instr Drivers palette. There are all the functions you need. Also, Go to Help>Find Examples>Hardware Input and Output>Instrument Drivers>LabVIEW Plug and Play to find several 34401 examples.
To learn about instrument drivers, go to the Instrument Driver Network.