how to use coins IN the game?

That is quite strange i must say. The moment you purchase anything from the store a little animation should pop up indicating the successful purchase and all items should automatically go to your in game credit.

The only instance where I was having trouble purchasing and recieving the IAPs where the game would refuse to connect with my credit card. But after switching between different credit cards, it started working again.

There is no mechanism in the game that blocks the usage of IAPs and you should be able to use all the bonuses immiedietly after purchasing.

Perhaps try completing the tutorial, maybe that will help, but other than that, check if your funds have been taken from your account. If so, and you still don't have the things you paid for, I'd contact easy tech (even though chances of them getting back to you might be slim or prolonged at best).

Hope this helps! Enjoy the game.

EDIT: i think I know what you mean. Coins and medals are in game currency to be used on purchasing generals and upgrading units and generals. The coins and medals you buy in the in game store, as placed in your "wallet" in the upper right hand corner of the screen BEFORE any game. During a game, the gold/material and gears you see in the upper left hand corner are gained from the cities and are not the same as the ones you use for buying generals. These are used to buy units on the game board and upgrade cities.