Step 1: Go into the spawn menu (Either by holding Q or pressing F1.)
Step 2: Click on the tab labelled "Entities"
Step 3: Scroll down left side bar until you see "WAC Aircraft"
Step 4: Click on it. (Preferably using left click. That one gives the best results.)
Step 5. Choose a vehicle and click on it.
Step 6: It should spawn . If it doesn't spawn, be sure to look at the ground and try again. (But not directly down, as it will spawn on you and kill you.)

Also, little tip for needing help with issues. Don't say "It's not working" when asking for help. It's incredibly vague and won't help anyone such as the other players or the guy who made the addon. For all I know, your issue can be something from not knowing how to spawn it or the engine not starting. Hope your issue gets resolved soon and you try to be more descriptive in your queries.

Sorry for this, it was my brother taking liberty of inquiring about this in my account, really no idea why because he has his own. well, he's a kid anyway so.
but good reply, hat off to you.

No problem. Glad I could help. Hope your brother doesn't steal your account again in the future and he learns how to use Gmod.