One way is to go to your adjustments tab and choose the Animal Companion adjustment. Once you add the adjustment to your Ranger class, you go back to the Ranger tab and there should be a new option to Add Animal Companion.
This currently doesn't list all the companions though, so most people work-around by just creating a second NPC hero for their animal companion.
I know that in the next community release I put in an automatic companion that auto-adds the ranger animal option when you gain the Ranger Companion ability, but I'm not sure when the next release will come out. You can go to
github and download a copy of the current pre-release files if you are brave.