Well are you plugging into dual 3.5mm ports on the computer (one headphones one microphone) or are you trying to plug it all into a single 3.5mm port using an adapter? If you are doing the latter, then the problem may be that. Many laptops have an individual port that is output only (not output and input, which is what is needed for a microphone).
If this is the case, you will either need to get a USB adapter to connect the headset/mic combo to and run it through the USB port, or get a headset/mic combo that comes with a USB connector.