Unfortunately, the Serial In pin on an 08M cannot be used as a digital input. The feature became available on 14M, 18M and 20M and of course, the M2 series of PICAXEs.
You have two options: upgrade to an 08M2 or, if this is your third input (Pin 3 and one other*), you could set up a resistor divider circuit and use the other* input pin as an ADC input. * "other" pin would need to be 1, 2 or 4.