Add-ons appear within the Research table, and require certain level districts, as well as resources to unlock. The cheapest and likely first add-on that will be able to be purchased is found within the "Economy" section of the research tree. It is called "Packing House Add-on" and requires you to first unlock it by having the following:
Scientific District Level 3
Industrial District Level 2
96,060 Food
96,060 Petroleum
75,576 Alloy
64,585 Composites
25 research points
Once you have that, you may purchase it.
Then, you need to purchase the "land" that it will be added to, which is found directly above the Industrial District within your base. This requires the following:
HQ Level 4
Minimal Resources
Once you purchase the land, you then need to apply the add-on to it by clicking on it and choosing to build the add-on which requires:
7,050 Food
7,050 Petroleum
5,377 Alloy
4,601 Composites
Click "Build", and it will take ~30 minutes to complete. Once it does, achievement unlocked!