How to call a function is explained in the tutorial. The LinearReg function and most other TA functions are called in exactly the same way as the LowPass function in workshop 4. If you are not sure what an indicator does, plot a line from it - then it normally becomes clear.

However, you're right - the LinearReg function doesn't make much sense to me either. Better use the LinearRegSlope function, which calculates the m value, the projected price change per bar.