I use DXF files on occasion.
DXF files must be converted to gcode before you load them on the tablet to be cut out. You can do this with Inkscape for many DXF files, but not all. If I come across a DXF file that Inkscape has trouble with, I use QCAD to convert the DXF file to an SVG file, then I use Inkscape to convert the SVG file to gcode.
Inkscape is free and runs on mac. QCAD is also available for mac and is effectively free for converting DXF to SVG (makes you restart it every 15 minutes unless you buy a license).
Here are the steps:
1. In QCAD, open the dxf file (File->Open)
2. Do a quick SVG export (File->Quick SVG Export)
3. Open Inkscape and set up for plasma cutting (see http://gofabcnc.com/inkscape/
4. Import the .svg exported from QCAD to inkscape (drag and drop the file into inkscape works)
5. Select the shapes you want to generate gcode for.
6. Click Path->Object to Path
7. Click Extensions->GCode Tools->Path to GCode
8. Click the Preferences tab and fill out "File" and "Directory"
9. Click the "Path to Gcode" tab and click "Apply" -- for some reason it gives an error message if you don't have the "Path to Gcode" tab open when you click apply.