The front is for the phone but you said your USB is broken on the phone so that is a no go. The back slot is for a spare battery (bare) to charge. This is how it looks:
http://cgi.ebay.com/NEW-OEM-Audiovox...QQcmdZViewItem
The cradle has the miniUSB connector so you can either use the USB-miniUSB cord to plug into a computer to charge or the AC plug-miniUSB cord to plug into a wall socket. Pick up some spare batteries. There are also posts on where to get the miniUSB connectors and how to open the phone to fix stuff. Don't know how adventurous you are. LOL.
Edit: Just looking at my cradle and documentation. You have to use the AC plug to charge with the cradle, so might want to invest in another one of those.
USB ports:
http://forum.ppcgeeks.com/showthread.php?t=24932&page=2
How to open 6700:
http://www.bellybuttonporn.com/wiki/...ur_PPC/XV_6700
This looks like just a battery charger but not OEM:
http://cgi.ebay.com/DESKTOP-AC-CHARG...QQcmdZViewItem