Was having the same problem but figured it out just now. If you look in your Device Manager and see the ! next to an ADB device, you need to download and install the HTC Sync application so it installs the ADB driver for your phone. After you install it, the phone will appear under Android USB Devices as My HTC in the Device Manager.
HTC Mobile Phone Support - HTC EVO? 4G (Sprint) - Downloads
Disconnect your phone first, install the HTC Sync, then reconnect your phone and follow the directions on that link again.