Make sure you are selecting the voicebank first. On the top bar go to Project (P) -> Project Property (R) and choose where it says Voicebank. Click OK.
Go to Tools (T) -> Voicebank Settings (S)
This window should appear:
Now you're diving into the world of oto.iniiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii!!!!!!!!!!!!
To start, each line represents a config entry.
Click on a line and look to the right side. In the Alias box you should only replace the hiragana with the respective romaji. Keep the spaces and the line and the suffixes (C5, G4, D4, etc) Also make them lowercase to distinguish it from said suffixes.
Make sure you press SET after each line! Otherwise changes are lost!
Make sure you press OK periodically!
Don't make the mistake of pressing cancel...