The reset input, /MCLR, is tied directly to the power supply, such that it is permanently inactive. In this case, we are relying on the processor's internal power-on reset circuitry and don't need to provide an external reset. It is common practice to use a pull-up resistor to tie an unused input, such as MCLR, inactive. However, in this case I have found that a pull-up resistor can affect the activation of the internal power-on reset to the point where it fails to kick in. (The internal capacitance of the pin combines with the resistor to form an RC circuit, which delays from reaching the appropriate level.) Thus, the resistor can actually cause the processor to never start properly. So in this case, it's better to leave it out.