The Toolbox is an often-essential part of the Enterprise Architect interface. There are several ways to bring up the Toolbox in Enterprise Architect. This tutorial will cover all 4 of them.
1. Via the Ribbon
Ribbon > Design > Diagram > Toolbox.
2. Using Shortcut Keys
Pressing Alt+5 displays the Diagram Toolbox.
3. Pressing the Space Bar
Brings up the Diagram Toolbox in-place via a context sensitive menu.
4. Switching between Docked Dialogues
If you like to dock the Toolbox and Project Explorer together in the same spot, the guillemets arrow can allow you to swap back and forth between the two.
P.S. A quick note about Perspectives...
Perspectives in Enterprise Architect are used to tailor toolsets for domain-specific modeling scenarios. So essentially, you can apply a Perspective to display functions/tools appropriate to the task you're performing and hide all the rest.
Sometimes people can't find the tool they're looking for because they have a perspective set and aren't aware. If you ever find yourself in this position, check the perspectives display down the bottom right of the program.
Then either select the new relevant perspectives required or set the perspective back to default by Perspective > Activate All.