How can I activate these IP in QSys?

Your target device has to be one of the device families listed for the IP.

I used one of the FPGA listed in the table to design my own PCIe card as the target device then that option will automatically activated?

Yes. You can also click the little gear above to make them available, but that means you'll have to move the generated files to a project targeting the supported device instead of using it for this project.

How can the QSys knows which FPGA I have used in the design? I used EP4CGX75CF23 in my PCIe card, why those items still in disable state?

System Settings or Device tab (depending on which edition of the tool you're using)

EP4CGX75CF23 is Cyclone V series, but why I can't see the item become enable?

That device may not support the hard IP, only soft.

If that device does not support hard ip why does it still in the list?