Controlling PSoC Registers
You can use the standard C way (note this is assuming a 32-bit register (e.g. PSoC 5 device).
PSoC also provides macros and typedefs for this process.
Register Description
Cypress fully describes all of the fixed PSoC 5LP registers in this PDF. Note that this does not include registers that are generated when you add components to the visual editor (e.g. UART and ADC registers).