This
link worked for me.
Install the LG Mobile Support Tool.
The application window will popup after install. To the right is a button to install USB driver. Click it.
Another popup window will appear.
Select your carrier and phone model.
Mine were T-Mobile and LGP509TN. (The last 2 characters refer to 3 color choices: Black, Maroon and Titanium)
Double click your highlighted device and the download will commence. The file is about 14 MB in size.
An installwizard window will automatically open. Select Next. A pop-up will tell you if it installed the driver pack correctly to your system.
Once you plugin your device Windows should automatically detect and install the appropriate drivers to your specific phone.
Hope this helps!