On the TC70/TC75 device, click the all apps option and click on File Browser. Navigate to the Storage --> sdcard1 folder. If you copied the file to the root of the device, scroll down and your .apk will be at the end. If copied to the Download folder you should see the Download folder in this same directory. To install the .apk simply click on the file and follow the prompts.
Note: The above applies to most Zebra Android devices.