Tap on the event, there should be an edit button in the popup. You can also tap and hold to drag events to different times or days. If set to day view you should be able to tap on the event to go directly into the edit window. You will not be able to edit subscribed calendars or shared calendars unless you have permission.
The edit option does not appear on the popup window. The calendars are the same ones on my computer, iPhone, and iPad. Those calendars can all be edited on both iPhone and computer so why not the iPad?
I wonder if it is an iCloud bug? MobileMe was not perfect, but since iCloud I have had nothing but problems with calendar and Contacts entrys duplicating themselves. In one case a calendar event duplicated itself into 93 copies. I loathe iCloud.
I had some minor glitches in the beginning with Note syncing but iCloud has been working good for me. I can see it being an iCloud glitch since the issue is on the iPad only. On my iPad, from the month and week veiw if I tap the event there is an "Edit" button on the right side of the pop up. From the day view if I tap on an event it opens directly to the Edit window.
If it is not working this way on your iPad I would Reset it first. From the Home screen double tap the Home button to view the multitasking bar at the bottom. Tap and hold an app till the icons wiggle with the minus button visible. Tap the button to close all apps then tap the Home button to get back to the Home page. Press and hold the Sleep button till you see the Slide to power off message and power it off. Once powered down completely press and hold the sleep button till the Apple icon appears.