If you're looking for your exam centre when you only have your ticket number or roll number, the short answer is that your institution or college should provide you with further details, or at least access to a database that will allow you to locate the correct exam centre based on the ticket number you have been issued.
Sometimes there are delays in this information getting to students, so it's advisable to check regularly whether the data has been published.
Personally, I'm not very familiar with the system itself, but if you take a look at sites like The National Institute for Open Schooling, then you'll see that most exam centres can be tracked down by searching an internal database.
