How do I install addons in construct 3?

I purchased a game template that uses a couple of addons and when I try to load the game from the my desktop it says there are addons that must be installed first but it doesn't tell me how :(

You should be able to install them using the the pane on the View:

Menu > View > Addon Manager > Install new Addon.

Ok, so are addons globally accessible between projects? I assumed that addons were specific to a project.

When you install the addons you can use them between your projects.

However, be aware about using them. I would avoid to use any third plugin party that are not strictly necessary, and are not from an author that is not active. The main reason is because they could crash over time (due the construct3 updates) and if the plugin doesn't get updated, you will end up with a broken project.