Did you try adding it manually?
- Run "Update Flight Simulator with new ATC capabilities" from the main menu of EditVoicepack
- Make sure "Change Settings" is checked on the first page in the wizard
- Press the button "Add Flight Simulator installation"
- Browse to your P3D installation and select Prepar3d.exe
- Select Open
If it still does not work, what exactly happens?
Can you find Prepar3d.exe? Do you get an error message when you select "Open"?