Not sure which aspect of your question is most problematic for you. I assume that you've looked at the concept diagram at
https://www.paypal.com/webapps/mpp/mass-payments
where the Excel worksheet is roughly analogous to a table's (or query's) worth of payees.
You would normally provide scripting code on your server/website that would submit that payee/amount list through the MassPay API calls against the PayPal website. If you only have one or two payees at a time, this is not the usual way to make a payment to your users (one-offs are generally Adaptive Payments API). It's not a downloadable app, though.
So is your app something the users download and interact with your site? If so, the correct place to put the code that faces PayPal (and actually transfers money around) is on your site's server. Not on the handset.