PC's can't read Gamecube disc's because Gamecube Disc's use heavy encryption and a special method which make them unreadable by conventional PC-CD/DVD drives except for a small handful of LG DVD-Rom drives which are able to extract a working image of a Gamecube game.
You'll need to use Google to find a method of dumping a Gamecube disc to your PC Such as USB Gecko and Phantasy Star exploit.
Or you can just avoid having to spend any money to try to play a game via Dolphin which most likely won't work and be extremely slow by taking the disc and play it on your Gamecube.
Dolphin is not ready to play games on and most games are still quite incompatible. You wion't be able to achieve playable speeds on Dolphin unless you have a decent PC with at least a 64-bit capable dual core processor and a 64-bit operating system.
Those are the required minimal requirements to play games on Dolphin at an enjoyable/playable speed unless the game(s) you want to play are 2D or 2D with minimal 3D geometry.
