Which API in Qnx is used to register the char device driver .please help me…
What do you mean by “register the char device driver”? A generic char device driver is a process. Once started, it becomes visible by taking control of some name space, ie. /dev/par. Maybe that is what you mean by registering?
how to install the driver
just run it 
its a mere process as any other process
If you want to run it every time the machine start, add it to /etc/rc.d/rc.local.
If it’s a driver for some sort of PCI device, you could edit the enum database to add your device to the list.
It implies that you have a QNX driver for your device or your device is supported by an existing QNX driver.