Where is Final Cut Pro X's Share Monitor App?
It is still available but buried in Compressor. Right click on the Compressor app and choose "Show Package Contents". Navigate the path shown in the graphic below to get to the Share Monitor app. Double click it to launch it or drag it directly to the Dock. Also notice the Create Disc app is there as well.
You used to be able to open the Share Monitor from a Compressor Droplet app and under Motion 5.1's Share menu, but the option is no longer available in either app.
Once you open the Share Monitor app, keep it in your dock for easy launching.
Click-hold on the Share Monitor dock icon and from the popup menu check Options > Keep in Dock
You can monitor active and completed encodes in Compressor's Active and Completed tabs, but I like having the option of using the Share Monitor to monitor my encodes independent of the Compressor app.