Hi and :welcome to Armaholic
You have to use a replacement pack together with the addon.
For example if u talk about this one as i think you are:
Here is the replacement (check that there are 3 diff replacements: heavy, mixed and light so you can only use one at same time):
They will replace the US soldiers i guess, so every time you play with those soldiers you should see the 1st Infantry Division ACU units instead of the default ones.
Not every addon has a replacement, you can check this section to see some of the replacements (remember you need the "original" addon too, the replacement along does nothing):
Some other addons are packed with their replacement, but u will have to find them yourself :D