Ever since Apple updated Final Cut Pro X to 10.3 back in October, a lot of casual FCPX users have found the new interface a tad confusing. Keep in mind, everything still functions the same, things are just in different places. One such change is the Background Task button, which has been moved to the upper left toolbar, it used to reside above the timeline, next to the timestamp.
Here are 3 ways to open up the background tasks dialog box:
Click the circle with the checkmark, which is located in the upper left toolbar. (Fast-forward the above video to the 0:11 mark).
In your Apple menu bar (with FCPX selected) click on Window, then scroll down to Background Tasks. (Fast-forward the above video to the 0:24 mark).
My preferred method, use the keyboard shortcut Command (⌘) + 9. You can also use Command + 9 to close the Background Tasks dialog box. (Fast-forward the above video to 0:33 mark).
For more information about Background Tasks, please check this Apple Support page.