Answer:
First you fill up the form for creating new ad-slot (button: Add sites). Then copy created ad-tag and paste it to your website HTML code. Your ad will be shown immediately. Do not modify ad-tag in any way as it could stop working properly and you will lose potential earnings.
Answer:
You can create as many ad-slots for one website, as you wish.
Answer:
Your earning depends on different factors. Main ones are: amount of traffic, quality of content and positioning of ad-slots. Ads located on the header will generate more clicks than ads inside. All your earnings will be automatically calculated and added to your general balance amount.
Answer:
Maybe you should increase minimum price for your ad-slots. And if it's not the solution, check the traffic has a good portion of unique visitors as these are ones we are counting only. If you have healthy traffic, advertisers will find you!