You should have downloaded the mods and now want to launch your mods and Arma 3 via Arma3Sync.

Head over to the "Addons" Tab and click the "Refresh"-Button at first.

Right-Click somewhere on the right side and choose "Add Group". You can call it whatever you want and then click okay.
Now you can drag and drop mods from the left side to the right side inside your group. This should be done according to the needed mods for the mission. (Usually found in the mission thread!)

You are also totally free to use any client side mod you want. These are mods that enhance the gameplay - like sound, graphics or UI - on your side without forcing other to use it. Some Examples are:

Be Aware that any extra mod can have an impact on your FPS

Please never use more mods (except those from our Recommended Repository) than necessary for the mission. It can cause unforeseeable issues!