Depending on the method you're using, this can usually be remedied in one of two ways:
- USB Cable: make sure your camera is turned off before connecting to the computer, and that the drivers for your camera have been loaded prior to connecting.
- Memory Card: SD or MMC card readers are becoming more std. on computers, or inexpensive card readers are available to connect via USB.
If you are experiencing difficulties with either of these, this may involve a problem with your USB port or the drivers your computer is using. Sometimes it's as simple as removing the driver which isn't working properly and re-installing it again. In most cases, you will need to install the driver without the USB device connected, so that after you have successfully loaded the driver, the computer will recognize your USB device properly.
Once again, make sure your camera is turned OFF when you connect it to your computer with the USB cable (of course, your computer needs to be turned on). Then, turn the camera on, and you should see the computer recognize the device.