The Japanese postal code usually indicates just the area, for instance, 102-0071 for 東京都千代田区富士見. However, large companies/organizations like publishers have thier own postal codes to save customers from writing out the addresses each time. 102-8584 is exactly the one for ASCII Media Works, Inc., thus, this code means 東京都千代田区富士見1-8-19 角川第3本社ビル 株式会社アスキー・メディアワークス, as you guessed.
「2011年8月号プレゼント」係 means the section to receive the application for presents in the August 2011 edition, and 行 is a humble title for thier return address.
This is a bit off topic, but applicants are required to draw two diagonal lines on 行 and write the correct honorific 御中 beside it as a Japanese business custom.