Launching third party file comparers in CS5/DW

First thing you need to do is go to the preferences (Dreamweaver/Edit --> Preferences --> File Compare) and select your app.  Then you have 2 options for comparison, if the files are both local, select them both from the Files panel and then right click --> Compare Local Files, the same can also be done for 2 remote files. Or if one file is local and the other is remote, simply click on the file that you want to compare and choose to Compare with Local(Remote) from the menu.