Not sure if you've already come across this: https://webapp.spire.umass.edu/admissions/htdocs/applicant/gradapp_logon.html
You need your Applicant ID and the Admissions access code, which were sent upon receipt of your application. Of course, I've been obsessively checking that all my materials were received, and it looks like at least one of my letter writers procrastinated too long for UMass! Probably the most frustrating part of the application process for me = the 4 or 5 e-mail "reminders" I had to send to my last letter writer (professor and thesis reader) before he finally submitted the darn thing!