In order to be able to type in Turkish:
Under the Apple menu (MAC OS X)click on System Preferences:
Click on International.
Then click on Input Menu (3rd one over).
Scroll down the list until you see Turkish. There are three choices for Turkish.
Choose 'Turkish' (not the QWERTY one). Put a check in that box.
At the bottom of the page check 'Show input menu in menu bar.'
A Turkish flag should appear in the Finder page on the top line next to your time and date and Internet symbol.
Be sure to also leave checked the native language you are using.

Close out of International

If want to type in Turkish click on the Turkish flag on the menu bar. The Turkish flag will now show and letters on your keyboard will type in Turkish letters. If you want to see where the letters are, open the same drop down list and click on 'Open Keyboard Viewer.' You will get a mini diagram of the location for the various Turkish letters. To return to your native language, click on that flag in your menu bar.