If you are leveraging the BIG-IQ tool (version 7.x or newer) to import devices using a CSV there are a few things you should know.
It would probably help to know the columns needed (note you do not include the column headers in the csv)
So when you Save your CSV it MUST NOT be in UTF-8 mode or it will fail when you try and import it with incorrect Management IP Address which is the first column since the CSV import can not read a CSV saved in UTF-8 mode.
Anyhow, your CSV should look something like this when your done
When you click begin you will see a very boring box while it tries to import the devices