The masspay file can only be generated for the last completed calendar month.
Otherwise, you can go to your Transaction Log, select the date range you want to cover and click View Log, then you can click Download Log to save that log view to a file on your computer. Unzip that file and open it in a spreadsheet program like Excel, then you should be able to sort rows by the Affiliate ID column, so you can easily sum up each affiliate's earnings during that period.
You may want to generate a monthly masspay file anyway, to follow as a template and modify as needed for the actual commissions you'll be paying out, so you can still upload that to PayPal Mass Payments. It's just a tab-delimited plaintext file, easy enough to view and edit in a plaintext editor like Notepad or textEdit, or a spreadsheet program if you wish. The format is pretty simple and should be self-evident, but this page explains in detail:
https://www.paypal.com/cgi-bin/webscr?cmd=_batch-payment-format-outside
Indeed, it occurs to me you could just generate a masspay file towards the very end of each month for commission earnings in the prior completed month, then just combine the files every two months and upload that to Mass Payments. Offhand, I'm not sure if they'll accept multiple lines for the same recipient in the same file, so you might need to combine amounts for the same affiliates into a single line each.
