Select "My Trade History & Analysis" from the Orders drop down
Click "Complete Trade History" to download your history
DO NOT open the file in Excel and "save as" a new file.
It's important to upload the raw file that Poloniex exports straight from your "Downloads" folder. This avoids rounding errors and corrupting the file.
Upload your trade history file into the Poloniex tab on Step 2 (Import Trades) and click the Import Trades button.