Here are some steps to try:

Drivers

In device manager you should have a device under "Ports" called "USB-SERIAL CH340 (COM*)"

Verify you computer has not installed the drivers

Open Device Manager by typing "device manager" in the start menu With the robot connected, verify wither or not an "Unknown USB Serial" device is showing. If its showing, then your system hasn't installed the driver yet.

Give Windows a second chance to install the driver

Sometimes windows won't download and install the driver the first time its connected. disconnect the robot and reconnect it again. Often Windows will install the driver the second time automatically. Verify in device manager that there is still an "Unknown USB Serial" device showing, and that if you unplug your robot, the device disappears, and if you plug it back in, the unknown device reappears.

Install the driver manually

You can download and install the USB Serial CH340 driver from http://www.wch.cn/download/CH341SER_ZIP.html After installation, dissconnect your robot arm and reconnect it again. Verify in device manager if the unknown device disappears.

Cable

If there is no device in device manager as "Unknown USB Serial" or a device under "Ports" called "USB-SERIAL CH340 (COM*)" when you connect your robot arm, then the cable may be bad. Swap USB cables with another one that you know works.

Power

Also, when connected to your computer, look through the bottom mounting holes of the robot arm to verify that there is a glowing LED, to insure your arm is getting power. When you do this, make your you don't have the external power supply plugged into the robot.