This is a “General” type plugin. So finding the feature is not in the GUI of a distinct Tool, Report, Gramplet or view. Instead it adds a new GUI element to all variants of a dialog. In this case, it inserts a “Recent items:” table in all the Object Selector dialogs.
Click on any share icon 
and the dialog with a Recent items: selector appears. (See glossary term: Shared Objects)
Since there are multiple items in the “recent-items” subdirectory, the easiest way for users to install is to browse to the gramps folder in the top directory in the kkujansuu repository and from the green <> Code button, choose Download ZIP. From the gramps-master.zip file in your Downloads folder, extract the recent-items to the gramps52/plugins/recent-items subdirectory of your Gramps User Directory.
The addon adapts to the version of Gramps and my test was with the 5.2.1 release of Gramps.
When selecting from the Recent items list, the main selector table focus (highlighting and scrolling) follows to the corresponding row.
After a bit of experimentation, some oddities appear.
- The focus for the Find a Record feature (aka: “interactive type-ahead search” or “seek as you type”) becomes a bit muddy with 2 active areas.
- The Citations in the Example.gramps tree did not scroll when the Recent Item was selected.
- The Select Father and Select Mother (from Family Editor) have a gender filter by default. But the Recent items doesn’t respect that filter. It cannot scroll to the hidden gender until you click the “Show All” checkbox.
But it is VERY useful. (Still need to experiment what the row limit is {most recent 6 rows, no scrolling} before the list turns into a scrolling box.)