How to keep my dog pen flea free?

I just bought a house with a 20×10 dog pen that has durable fencing for "walls" and a completely dirt floor. She used it once and got fleas! We live in the country, surrounded by farms. What can I do to keep the pen area and the dog house inside flea free? Please keep in mind I give my dog Advantix often, as prescribed and she still got fleas…

just try to put stuff on the outside of the house to keep the pen away