When installing multiple screws into a single device like a CD-ROM, insert all the screws before you tighten any of them. This lines the device up and makes it easier to complete the install. If the cable doesn't fit, just shave off the edge, and if it does fit, then obviously it is the right port. If the cable doesn't fit, then find out, from the manufacturer if an adapter is needed, or if the device is even meant to be used in your computer. More importantly, don't assume that because the cable fits, then it will work if you plug it in.
Some cables have the same form factor, but are designed for completely different interfaces.
Tips- Many cables are keyed to only fit one way. If it doesn't fit, turn it over, and try it the opposite way. In some cases manufacturers try to abide by a standard to make life easier. This is the Ground line, and always goes to the Ground Pin, Pin 1, which is almost always situated adjacent to the power connector. Finally, many computer manufacturers are now incorporating cable guides into their case designs.