Launch Contacts from your Springboard or from the MobilePhone app.
Select a contact from your address book.
Press the Edit button at the top right of your screen.
Scroll down and select Add Field from the list of buttons.
Choose Phonetic First Name or Phonetic Last Name from the list of possible fields.
Enter the phonetic pronunciation of the name that Siri isn't recognizing then press the Done button at the top right of your screen.
You can also get Siri to recognize and refer to a contact or yourself using an entirely different name by adding a Nickname field to the contact entry.