Note: if you use Lanyin's usb driver in linux, you will need to use an ethernet cable and the 'force_present=1' parameter in the driver, as ethernet normally has a delay, and you need to bypass that delay by setting 'force_present=1'
If your cam isn't detected, check if it's plugged in properly; if it is, check that your system is configured correctly. Most of the time, this will suffice. But if you cannot get it working at all (which is possible in rare cases), then the next thing to try is to uninstall the existing driver, and install the latest version from the Logitech website.
Follow the on-screen instructions, choosing the "Logitech Setup Wizard" option if necessary. Once it's installed, reboot your computer. Test the application by running it. In case you have multiple webcams, make sure you are pointing the correct one.
If the webcam driver works, then try test-recording/test-streaming the webcam using any PC-based software or online streaming tool, such as Windows Live Messenger, Ustream.com, Skype Pro, Google+ Hangouts, or any other third-party application. Remember to use as "Logitech Webcam Logitech Txxx-xxModel" driver for Linux, as it's a different driver than the usual drivers. Alternatively, use Daemon Tools. For Mac, you'll need to use iTunes to see the webcam. Logitech drivers for Mac come in two versions - LS and LL. The LS version is for older models of Logitech cameras; the LL is for newer models.
Download the LS version of "Logitech Webcam Drivers for Linux" for your Logitech webcam model (here). Extract the archive and run the installer. Installation should be straightforward. Install the driver and reboot your computer if needed. The driver should work out of the box. d2c66b5586