How to add coins (virtual currency) to a member after posting a question?

First, make sure you have installed14 and enabled15 the Virtual Currency app. 

Then go to the user's profile page> click on (...) button on the top-right> Add Credit> Inset Amount and description> done:

You also have the option to "Deposit" credit, which means you should have added credit to your own account first and then you can transfer it from your admin account to the user's account.