Sometimes we close the SSIS toolbox window by mistake, or may be due to some reason it is not showing then you can open the SSIS tool box window using approach mention in this article.
When you create a new package, By default SSIS tool box window can been on the left side as shown below.
Now, in case if it is not showing for you or you close the window by mistake as shown below.
Open SSIS Toolbox window
To open the SSIS Tool box window, In package design tab, next to Package Explorer you will see two icons, click on second icon to open the SSIS Toolbox window.
Another way to open the SSIS Tool box window, click on View tab then navigate to Other Windows then select SSIS Toolbox.
you can see, a SSIS Toolbox window open.