If you check-mark the box beside the mountpoint (/data), it mounts, if you uncheck it it dismounts.... no other buttons needed.
I'm running TWRP 3.2.3, and when I boot into it, /data is already mounted (by default, TWRP mounts /data, /cache, and /external_sd), and I can run the terminal and do "ls /data" as a quick test to see its contents and verify the mount-point is working. I even did "ls /data/media/0/SuperMan" as another sanity check.
So, make sure the /data mount point is check-marked, then go to the terminal and do "ls /data/media/0/SuperMan" and you should see the EFS backup directory listed.
If that doesn't work, there might be something wrong with the partition that mounts to the /data mount-point, which is /dev/block/sda18 ..... you can see what partitions are mounted where by simply issuing the command "mount" in the terminal....... if you don't see /data mounted near the end of that output, you might look at "ls /dev/block" to see all the block devices.... you should have sda18 in that list (last partition gets all the leftover space for /data).