I have installed the plugins but for configuration i can't find item-loop.php file to paste the code. I am using gamma theme kindly tell me the alternative solution. Thanks.

loop-single.php is what you meant? Favorite items plugin in gamma does not need theme modifications.

Thanks for the quick response!
Actually, the plugin isn't functioning properly only button is appearing, and there is an instruction in the plugin that to configure plugin paste the following code into item-loop.php file(screenshot attached). But there is no item-loop.php file present in the gamma theme.

@arshlan
Favorite items plugin in gamma does not need theme modifications. It is pre-integrated already.

Why it is not functioning properly. It should even work without signup...
Have a look at my website 10cedis.com

Solution is in thread with you question why user rating is not working. Reason is same.
Continue in that thread.