You should not need to disable the onboard graphics, when you add in the add-on video card, the motherboard should automatically switch to that. Read this
https://service.msicomputer.com/msi_user/support/techfaqdetail.aspx?formid=3136
How do you know the video card runs fine if you don't get any video output from it? Did you test it in another system? Did you try a BIOS update on your motherboard?
CX is one of the worst power supply lines from Corsair, test the card in another system, if it works there, try yours with another power supply. Almost every single post where there is an issue with a video card not running has a CX power supply. It may not be the issue for you, but you need to check that as one of the first things.