Hello, In any PSoC3 project, the device settigns are performed through what is calle "Design Wide Resource"(project.cydwr) that can be found in the workspace window. In the CYDWR file, you will find a Clock Tab. In the clock tab, you will find a "Edit Clock" button which will bring up the Clock Tree. This is where you configure the system clocks. The clock that is used for system operation is the "Master Clock". The master Clock can be derived from multiple sources(PLL, IMO, XTAL or external digital signal). Typically IMO(Internal main oscillator ) is fed to the PLL. PLL is locked to particular frequency you need and PLL is used as main clock. Using PLL it is possible to operate the device up to 67Mhz.
I strongly recommend you to go through the tutorial available in this webpage, to understand basics of PSoC device and PSoC Creator.