The line looks ok but the setup is wrong.
You need to create that 'units' folder in the root of your ArmA installation. Say you have installed ArmA in c:\games\ArmA then the path would look like this:
Next, inside that @units folder you create a new folder called 'addons' so we have:
And in that addon folder you extract the .pbo files from your addon/mod.
I made you a screenshot how it looks here:
Switzerland was faster....