The link can be found here. The path to this page is Log In>Billing Tab>Installment Plan.
But if you're like several of us, you won't be able to see the details, but rather an error message. This is a known issue and it's been this way for some time now. If you have specific questions or issues, you might want to reach out to T-Mobile directly by dialing 611, using the Message Us link on this page, or by contacting T-Force via Facebook or Twitter. (Or if it's not necessarily account specific, you can throw out a question here and we'd be glad to help!)