Please try following and see if works
1. Download Jdeveloper from otn
2. Run installer to install Jdeveloper
3. Launch Jdeveloper from command line: \jdev_home\jdeveloper\jdev\bin\jdev.exe
4. Install adf mobile extension from menu help/check for update... or from local file
5. Re-launch Jdeveloper from command line again (as in step3)
I have successfully installed ADF Mobile extension with JDeveloper version 18.104.22.168.0. I had to download Jdeveloper again and then tried the same steps.I think there was some issue with Jdeveloper that i had downloaded earlier though it was the correct version(Studio).