Usually when you download the addons there will be a readme or instructions on the download page as to where to find the units ingame.
Maybe you didn't install the addons correctly. Ok, well you put the PBO files inthe addons folder or a mod folder (recomended). When you extracted into the addons folder you put the PBO files from the folder you extracted (if you extracted a folder) in the addons folder and moved them in the addons folder? You really should just read the FAQ
and make sure that you're doing everything right, installing addons is a pretty simple task, just take things slowly if you're not sure. The instructions are clear enough and it is not recomended to install addons in the actual main addons folder.
Reasons to use modfolders:
When an error occurs it will take you ages to find all the 3rd party addons and remove them 1 by one finding the error.
When you patch arma it may be corrupted if you have 3rd party addons present in the addons folder.
Mod folders can easily be disabled and re-enabled for whenever you want.
Some servers don't allow some addons, others require a select few, meaning that you select a mod folder for a certain server you might want to play on.