Can't observe DC offset in a simple RC circuit with an oscilloscope
You have constructed a simple form of high-pass filter. The impedance of the capacitor rises with decreasing frequency, and at DC, it is infinite. Which makes sense, since a capacitor is essentially two metal sheets separated by an insulator &mdash; it clearly isn't going to pass a a DC current.

For AC frequencies, the impedance of the capacitor forms a voltage divider with the resistor. There is a frequency at which they are equal, and this is known as the "cutoff frequency" of the filter. Below this frequency, the output level of the filter drops approximately linearly with frequency. Above this frequency, the output level gradually approaches the input level.