How to access FPGA Manager's Input & Output ?

Have you tried running that code with your finger on the button? Often board designs make push buttons active low so if you read 0xFFFF_FFFF when nothing is pushed and something like say 0xFFFF_FFFE when you push button 0 then it's behaving as I would expect with active low buttons.

Last but not least are you 100% sure those buttons and LEDs are connected to the FPGA manager GPIO and not just HPS GPIO? If the buttons/LEDs connect to FPGA I/O and are wired up to the HPS through the HPS Qsys component then yes, that's the FPGA manager GPIO you are using. If those buttons/LEDs connect to HPS I/O then you want to access the appropriate HPS GPIO component registers.