Place the file unitconvert.m in the applications directory of Mathematica. In windows it is %user/Appdata/Roaming/Mathematica/Applications. Restart Mathematica after placing the file in the directory.
Call the Package using
<<UnitConvert`
The simplest method is call the function
getUnitsInterface[]
This displays a GUI with controls for the user to enter the Quantity that needs to be converted.