Do you have SuperSU installed? I would open SuperSU and look under the Apps tab to see if StickMount is registered, and if so, click on its entry to see if has been granted (superuser) access rights. If not, you should be able to grant access at that point.
Also, on my tablet, the USB drive was mounted on /storage/sdcard0/usbStorage/sda1 rather than on /mnt/usbdrive/sda1, where I expected to find it.