Hi Majdi,
There are 3 examples for PWM in the TIVAWare installation
TivaWare_C_Series-2.0.1.11577\examples\peripherals\pwm
You can refer to the invert.c example. Do put a while(1) before PWMOutputInvert API call so that you can see the 250Hz signal. This example can then be modified for the 60Hz PWM.
Regards
Amit Ashara