The code in the WPI documentation tends to be for Java, not C++, and it looks like that's what you've written here. (At least on the third line.)
Notice that m_leftShifter is of type DoubleSolenoid*. That means it's a pointer to a DoubleSolenoid object, and so to access methods of that object (like Set()) you need to use the arrow operator (->).
Try this code instead:
Code:
DoubleSolenoid *m_leftShifter;
m_leftShifter = new DoubleSolenoid(LEFT_DRIVE_LOW,LEFT_DRIVE_HIGH);
m_leftShifter->Set(DoubleSolenoid::kForward);