This is the method I use to design in Inkscape and export to RDWorks which gives good results. Note that these instructions are intended for vector cutting. For a raster engraving you can just export to a raster format like bmp, jpg, png, etc.
First, I like to setup my design in mm. This isn't required but I find it's easiest to design and work with mm. To do this, go to File->Document Properties in Inkscape and set the default units to mm.
Next, You need to make sure that all of your design is a Path. To do this, select everything and go to Path->Object to Path.
Now you are ready to export. Go to File->Save As. Give your file a name and select "Desktop Cutting Plotter (AutoCAD DXF R14) (*.dxf)" under "Save as type." Click Save.
You will then be presented with a dialog with some options.
Next, click OK.
Inkscape will then save the dxf file. You can bring this into RDWorks by going to File->Import and opening the dxf file that you just saved.