You can quickly add a new milestone in Teamwork Projects no matter which part of your site that you're currently in, by using the Quick Add button in the top right above the search bar:

You can click the 'Add Milestone' option, or to speed things up even more press 'Q' on your keyboard to bring up the menu then press 'M'.

When you choose to add a milestone in this menu, you'll need to pick which project you want to add it to first, then you can add all the details of your new milestone - all without leaving the screen you were viewing: