gius,
This has not much to do with oximeters possibly, but with your use of the API it provides,
You should study their docs. For example I found this:
https://github.com/Medisana/vitadock-api/wiki/Bluetooth-Access-of-the-DevicesAnd some other Github stuff. It seems that they have an app that collects the data and that you can process these data from there, for which you may need an access token and for which you need to write a very different type of app. It seems though that direct access to the devices is possible, but that they will not provide support.
This is totally beyond my expertise and has not much to do with APP Inventor.
Anyway, you did not tell why you did not receive any data and what you tried. Show some blocks!
Cheers, Ghica.