Take a look at your attributes palette when you select the type. It could be the text was set to overprint, since there is no color applied in the fill, nothing appears in the file. Make sure you did not select "no fill" for the letters, there will be nothing in the file. Instead, select the White swatch in your color palette. There may be some type of color applied to the stroke, that should left "no stroke". There also may be some fringe thing going on in your transparency dialog, check that as well. If the text is on a layer, make sure the layer is set to print. You could flatten the file to simplify things.
Thanks, but I've checked all that. In Illustrator the letters are filled with the white swatch, no stroke, and no overprint fill. There is no transparency and opacity is 100%.
It seems that the white fill gets turned into "no fill" in DXF, DWG.
Make a new White swatch and make sure it is set as a Spot (not CMYK) and use that for a fill.
Spot white color didn't work either. However, I think I've figured it out.
I changed the default export setting from "Maximum editability" to "Preserve appearance" and it works.
Not being familiar with AutoCADD, I thought it was important to use Maximum Editability. However, in Preserve Appearance the vectors are retained, so it seems it does no harm.
Even though you figured this out yourself it is a good subject to be known and you should mark the thread as answered.
So you can mark the thread with not awarding points unless you feel that someone else was helpful. In either case this might come up again and having it marked is good.
I tried to mark this answered, but there is no box in my own post to mark it answered. This is in the middle of a busy working day, so I lost patience with the interface.