Thanks for your help,
the first problem is gone but it is still not possible to export to dxf.
here is the message I get when I try it with the dxf export plug in:
19079 faces exported, 0 lines exported, 38992 objects ignored.
Is it possible, that my model is some how to big to export?

Thanks Dave, the first problem is gone, now I have the edges back.
But I still cant export them. I used the stl/dxf eport plug in.
here is the message I get when I try to export it: 19079 faces exported 0 lines exported 38992 objects ignored.
Is it possible that the export volume is so how limited and my model is just to big to export?
Thanks for all the answeres so far!

Hello every one,
I just have a strange issue after using the rount corner plugin.
The plugin worked perfectl well for me and all the corners I wantet to be round are round now. The only problem I have is, that there is no edge or line left, where the round corner is. It seems that the two faces I rounded up are now one big face.
Now I have a big problem when I try to export my 3d object to dxf.The problem is that it does not export the round corners .
When I press the right key on my mouse and try to select only the frame edges of the model, Then Sketch up will select them and show them in a doted line. But I can not clic on them or fade them in. So what should I do to make these edges selectable and exportable to dxf?