The DMT file extension is associated with DeLorme's XMap GIS and Street Atlas applications with GPS data collection forms. DMT file extensions contain transfer data used for internal purposes within DeLorme products.
At this time, DMT files are not supported by the ArcGIS Data Interoperability tool and cannot be opened in ArcMap.
To import .dmt files for use with ArcMap, Esri recommends converting the .dmt file to the KML format.
Subsequently, the KML file can be imported into ArcMap and converted into a layer by using the KML To Layer tool found at ArcToolbox > Conversion Tools > From KML.