Hi Manuella Lange
If you wish to convert an .sdlxliff to .tmx, the way I would do it is to create a new translation memory (.sdltm) in Studio via Memory Management > New.
Then import the .sdlxliff via Memory Management > Highlight the TM in the left hand pane, right click and select 'Import' > Navigate to the .sdlxliff you wish to import (you need to know where it is to do this) > Import
The resulting .tmx will contain only the contents of the .sdlxliff you wished to convert to .tmx.
There are other ways to achieve this, for example using 'batch tasks' but I think the above is the simplest. So long as you know where the .sdlxliff is on your system (usually in C:\Users\YourUserName\Documents\Studio 201?\Projects)
I hope this helps,
All the best,
Thanks Alison. Your help is really appreciated. Best, Manuella
You're very welcome Manuella!