Plus the trick I figured out (with help from Google!) - your bike has to be running for the PC III USB to actually transfer data between it and your laptop (unless you have the PC III USB powered by a separate 9V battery). Make sure to save your existing map from the PC III USB to your laptop before installing a new map - just in case you want to put the original back in.
Also, stick to maps that are specific for you sub-generation of bike. Mine is a 2002 so only 2002-2003 maps will work properly. The next range is 2004-2007 or 2009 or something like that.