sumancholleti,
Let me confirm that MyEclipse doesn’t provide an integrated support for Flex. Now coming to the problem on hand, I need to understand your current configuration. Can you copy and paste your installation detail? You can get it from MyEclipse > Installation Summary > Installation Detail.
Can you document the exact steps in trying to install flex plugin? For e.g, if you were using an update site, can you specify the URL? If you have downloaded flex plugin, can you specify the path to the download? How did you install the plugin, etc.
Also if you have any plugins installed already in MyEclipse, can you list them for me? You can go to Help > Software Updates > Add/Remove Software.
Thanks for your help.