Hello there! ^_^

I'm sure you have either figured this out by now or moved onto a different payroll solution. Whichever the case, I wanted to post the solution I found, in case it's helpful to others. It's quite simple and I understand everyone's frustration on this, as a bookkeeper who just migrated 10 years of historical data for a client from Desktop to Online. 

Try the following if you are unable to create your IIF export file from Payroll Settings:

1.) Make sure in the payroll settings you have selected the correct export settings for either Windows or Mac by selecting the little pencil icon on the right. Ensure your accounts match precisely with where you want the data transferred in Quickbooks Desktop.

2.) Now, click Payroll on the left main menu and be sure you are in the Overview tab. 

3.) Moving down from Overview you should see a link titled "View Paycheck List". 

(If you do not see it you can click on the Employees tab to see the link again toward the top right of the screen underneath "Run Payroll")

4.) Once you've selected Paycheck list, ensure that the pay period/date range you would like to export has been selected correctly. 
5.) Here comes the Magic Moment - Instead of looking to the right where you see "Share, Export to Excel, and Printer Friendly," Just select the empty box at the top left of the report to select all line items.
6.) You should see a button labeled "Export to accounting", where you can download your paycheck IIF file as many times as you'd like. 

If anyone found this helpful, please give it a "cheer" or thumbs up so that others see it quickly.