Are you doing any TSL dialog editing? If not, then you can safely ignore that message and work with KotOR dialogs.
There are two different dialog.tlk files for the two different games and there are features in TSL dialogs that are not supported in KotOR. Note that dialog.tlk files are simply the master list of every line spoken in the game but have no tree-branch organization. For that you need .dlg files.
To view a dialog tree, you have to extract the *.dlg files from the RIMs and/or from the template.bif.
See dlgeditor_readme.txt (section: Using DLGEditor) for more information.
Further questions regarding DLGEditor should be directed to the T3M4 forum.