The import file is, now, supposed to be a CSV (Comma Separated Value)
Meaning the file should be a text file purely.
It should contain the information as:
MACWITHCOLONS,HOSTNAME.
Your file, appears to be in the wrong format with:
MACWITHCOLONSHOSTAME
So the number of cells is invalid. Place a comma between the name and mac addres and remove the header field and you should be good.