One thing to be sure of, if you’re converting an existing product or working with imported products:
When editing the Product, make sure that “Remote Service” is checked (selected) under “Product Categories”.
This is a unique category created by the StockUnlocks plugin that activates the unique fields necessary, including the IMEI field.
You can see the explanation around the 1 minute mark in the first video for the StockUnlocks plugin for WordPress:
https://youtu.be/GhQkAgmOyZc?t=1m7s (WordPress: StockUnlocks Plugin Installation – Mobile Unlocking)
Also, if you’re attempting to modify an existing Product that you had before using the StockUnlocks plugin, check out this link:
Hope this helps!