Mac is terrible for add-ons. To my knowledge you must port them for mac if your going use them on mac. Better get windows.
Holy stuff you are stupid. (You figure out why.)
ONTOPIC:
Here I'll post pictures. I use a Mac as well :)
Find the add-on you want.

Click the Download button.

See I haz the archive. :3

Find your add-ons download location, and open the Blockland folder. Then drag that .zip into the add-on folder.

Make sure it is in the folder.

Start Blockland and select, Start a Game.

Select add-ons on the bottom of the window and select the add-on.

Start the game, plant a brick, and wrench it.

Find your add-on, and enjoy :)

If that doesn't work, then maybe you have 2 Blockland directories?