Hello Evelyne Ridyard 

is the File type selected - box ticked?

Daniel

Evelyne Ridyard 
You could try a workaround using InCopy: In InDesign, you export all Stories to InCopy, save those in a separate subfolder (as usual) and import them into Studio using the InCopy file type. Once you replaced the original InCopy files with Studio's output you will see in InDesign under Editor-Assignments that the linked files changed and you can update them, thereby importing the translated text into the indd or idml. It works really well, but if you've never worked with InCopy before it would be worth reading up a bit on how InDesign and InCopy connect - they are made to be used together, but there is a slight learning curve. Hope this helps.
Daniel