The short form answer, if you have a standard VCD disc, is that you can't play it under X, period. There's a bug or lack of support in X's CD driver that won't allow it to properly read data from VCD formatted discs. It's getting better--at least 10.1 will allow you to mount the disc--10.0 wouldn't even recognize it.
If you've got the .dat file from the VCD (for example, by copying it while booted into OS9, which can handle VCDs), QuickTime may be able to handle it, and if it can't Rosetta (X native) or VCD Gear (great program, and free, but classic only) can clean it up into an MPEG file that QT will play. As for that MacVCD program, it's more or less useless under X--you have to use the OS9 version, while booted into 9, to get the file off the VCD, then burn it onto a Mac-format CD which won't play in a normal VCD/DVD player but X will mount and read, allowing the X MacVCD app to play it.
Your best bet is booting into 9 or waiting for Apple to support VCDs.