You'll find it in the top right of your screen, next to the Quick Add menu:
In the Switch Project menu, you can search for a project if you know the name, or you can use one of the three tabs to help you find the project you're looking for easily:
This tab will contain a list of links to all your projects in alphabetical order, showing if they are 'starred' or not to the left of the name and showing the owner company to the right.
This tab contains a list of links to all of the projects you have starred (you can do this by clicking the star next to the project name in the top left of the screen from within the project, or in the projects list area). They are in alphabetical order by name, and will show the owner company to the right of the name. This means you can star the projects that you use most often so they are easily accessible from here.
Here you'll see a list of up to 5 projects that you've accessed most recently, with the most recently edited one at the top. You'll see a note of how recently it was worked on under the name, beside the owner company of the project.
You can also use the menu directly from this tab to access the project sections directly (tasks/milestones/messages, etc.), or to access project settings by clicking the down arrow menu icon to the right of the project name. Or, just click the project name to go straight to the landing page for that project.