Install the LG Driver first. If you don't have it, you can download it here:
http://www.lgforum.com/resources
When you connect to your PC. the phone will prompt you to choose the data mode you want. Choose Mass Storage and it will show up under My Computer and allow you to copy files to and from the the MicroSD Card. It won't show up on the Desktop of your PC, just under the My Computer Icon.
If this isn't working, then try another USB Port, or unplug and plug the phone in again. Source