The site will first ask you to provide your city of residence. After you enter the name of your city, the website will then provide you the addresses of available test centers in your city and in the cities which are closest to your city (looking along various directions).
You can select up to three centers to check availability. Once you select your preferred centers, you will be shown another page which lists your preferred centers. The first center in the list is active by default and is paired with a calendar-like interface which displays, in green, the days on which slots are available at the selected center. Click on any of such days marked in green to see the slots available for that day.
To check slots at another center in the list, click on that center to make it active. This causes the page to display the calendar interface for that center.
You can take the GMAT five times within a 12-month period. For example, if you take the test for the first time in the month of June of a particular year, you can take the test only 5 times before June of the next year.
Further, there has to be a minimum gap of 30 calendar days between tests. For example, if you take the test on the 1st of June 2014, you can again take the test only on the 2nd of July 2014.
At the time of registering to retake the GMAT, if you provide the date of your most recent test, the website will tell you the earliest date that you can retake the GMAT.