Go to Currency Pricing Conversions and if you didnt fill out a payment processor yet, it will redirect you to do so. Return to this link once you have.
If you havent filled out any pricing tiers (a "conversion"), you will be prompted to do so. You set the appropriate conversion rate for one fake currency to real currency. Leave blank the ones you don't want to accept (sounds like you just want Euro)
You can use the minimum/tiered option to create additional price points and price breaks depending on the amounts people ask for. Save it.
Next go to activity amounts configuration, and if you dont have an event using the Purchase action, add one for the currency you setup the conversion with.
Fill out the event as you see fit, although simply saving it as it is presented to you will cover the basics. Save it.
Now anywhere on your forum where currencies are displayed, click the number to open the currency popup.
From there, you can choose purchase from the From Account dropdown, and type the amount you want to buy.
The price will update automatically and you submit when you are ready to buy. Complete the regular process through the payment processor, and when you have returned to the forum, you will have the purchased credits.