Adding Indian Currency in Opencart is really simple. Just follow the steps below it will hardly take 3 minutes.
For Adding Indian Currency INR
System > Localisation > currencies
Currency Title - Indian Rupee
Code - INR
Symbol Left - Rs.
Status - enabled
Now Changing INR as default default currency ... Go to
System > Settings > Local (Tab)
change Country, Region / State, Currency
I think its very simple, this was succesfully tested on Open Cart 2.X.... , if you have any problem then let me know through comments or register on my website to discuss.