Decoupling feedback for testing of PWM controller IC for SMPS
The documentation surrounding the KA3882 is pretty poor compared to devices such as the UC3843 made again by Fairchild, ON semi and TI. So make a comparison: -



Make that comparison against the TI UC3843: -



You can also note that the pin numbers are the same and you can pretty much assume that functionally they are the same: -




  What is the purpose of I-Sense current in a PWM controller IC?


It's protection against too much current being fed through the inductor and it saturating excessively: -



So, to get more information functionally about the Fairchild KA3882 I'd look into the TI UC3842/3843 range of devices.


  I skip applying a current to this pin, but I am not sure and need
  advice.


Don't apply a current input - it requires a voltage that represents current BUT, you can connect it to 0 volts for just a bench test.