It isn't clear to me where to connect an external oscillator to. The PSOC dedicated MHz osc pins appear to be used only for an external crystal.
The oscillator of interest is in the 10-20 MHz range, can drive a CMOS load of 15 pF, with a rise/fall time of 6nSec.
To use an external clock source, please follow these steps:
1. Place a digital input pin in your top design.
2. draw a wire from that pin and give some name to that net.
3. In the .cydwr window, go to clocks tab and double-click 'master clock'.
4. A window named 'configure system clocks' will open. In this, check the 'Digital Signal' and click on the browse button.
5. Then select the net that you gave for the pin (clock input) previously and give the frequency and tolerance value.
6. Now you can use this clock to derive any clock source.
For further information, please refer to the application note available at