The Blimp will unlock once you reached a population of 5500.
Step 1: Select Transport. Buy a Blimp Depot ($20,000) and at least 2 Blimp Stops ($16,000).
Step 2: Select Blimp Pathway to create a route between the Blimp Depot and at least 2 Blimp Stops.
Step 3: Press
on any Blimp Stop to Create a new line, go to another Blimp Stop and press
to add a stop. Go to the Blimp Stop where you started the line and press
to Complete the line.
Repeat Step 3 another 7 times and the achievement is yours.
After the 3rd time you will also unlock the blimp blimp achievement.