I am unable to see any extension installed in magento in Magento Connect Manager.
Can someone help?

I am unable to see any extension installed in magento in Magento Connect Manager.
Can someone help?

This could be a number of things, but it's most likely a missing, corrupt, or impossible to write to cache.cfg file.
When you install an extension via Magento Connect, it's value is saved in the compressed
downloader/cache.cfg
file. My guess is this file is missing on your system.