The feature has to be turned on under Settings > Computer Management Framework Settings > Application Usage.
It doesn't get enabled by default, but during initial JSS setup it will ask if you want it on or not. Either way, until that's on you won't be able to pull this report. Even after you turn it on, you're going to need to give it a while (read: days / weeks) to gather the data before you can pull anything useful.
Edit: Just curious, but any particular reason why you guys are still on JSS 8.1?
After upgrading from 8.73 to 9.3, we lost all of our application usage log data. We had been collecting it for over a year. For example, Logic Pro X had over a hundred logged computers using it over the past year. Nothing after the upgrade, I'm only seeing a few entries after nearly a week of data collection and the rest in the 'Computers without logs' area. Fortunately, I have a backup of the db from before the upgrade, so we can either extract that history or downgrade, but this seems to be a problem. Also, in 9 there's no longer a way to adjust the time frame when viewing the logs and no handy "create smart group" capability in the unused license area.