It’s not posted on their site yet – http://www.garmin.com/support/download.jsp#programs – but you can update to 6.7 from within MapSource by clicking on Help / Check for Software Update.
Here’s what is listed between 6.5 and 6.7…
1. This will be the last version of MapSource to support reading/importing PCX5 files. For future compatibility, please migrate your data to the GPX format.
2. Added route road and area avoidances. To access avoidances, select “Manage Route Avoidances” on the Routing tab of the preferences dialog. Once avoidances are created, they are displayed on the main map if appropriate for the current map product.
3. Added “Auto” option to the display of route avoidances. This will hide route avoidances when the map is zoomed out far enough.
4. Added new “large” 24×24 pixel waypoint and map symbols. Select these symbols on the Display tab of the preferences dialog. Custom waypoint symbols can now be 24×24 and 32-bit color as well.
5. Added the ability to show or hide the left hand (user data) pane.
6. Added tooltips to the waypoint symbol selector with the name of the waypoint type.
7. Added celestial information dialog.
8. Added schema extension to GPX for handling Garmin-specific data elements, such as waypoint depth. The location of the schema is documented in outputted GPX files.
9. Updated the GPX Garmin extension (GPXXv1) to version 2 (GPXXv2) based on comments. GPXXv1 will not be supported.
10. Added ability to manually set the Y-axis endpoints on vertical profile plots.
11. Added “rest area” find subcategory.
12. Added artificial reef and oil rig map symbols.
13. Removed real-time tracking from MapSource. Use nRoute for PC real-time tracking applications.
14. Made several map rendering performance improvements.