thanks for the offer... it is 4AM... but I'm happy.
The trick was done by loading the model (collada DAE) into Blender, assign the textures to the model and export it as 3DS.
(The plugin is still there and can be activated, even though there is a warning).
After that ISADORA recognised the textures and applied them properly so I could see the model.
As noted in previous posts in this forum I had to rename several of the texture files so the names were short enough not to cause any problems.