Press the "MODE" button until LCD dispaly shows "TTL BL".
Additional information on TTL fill-flash can be found on page 33 of SB-600 manual which states :
TTL auto flash mode: TTL
In this mode, the flash illumination that is reflected back from the subject is detected by the camera's TTL auto flash sensor and the camera automatically controls the flash output level to give the correct exposure.
Automatic Balanced Fill-Flash: TTL BL
Press the "MODE" button to display "TTL BL" the LCD panel, and Automatic Balanced Fill-Flash is performed. The flash output level is automatically adjusted for a well-balanced exposure of the main subject and background. When performing Balanced Fill-Flash, however, no "BL" appears with cameras in Groups III and IV.
"TTL BL" indicates i-TTL Automatic Balanced Fill-Flash in the i-TTL mode.