Hi Bipin,
This usually happens because yum-updatesd is trying to update your
installation, in the background.
Either you can wait for it, to do it its thing, and try again a little later.
Or you can stop the service manually.
You do this by opening a terminal and typing
$sudo service yum-updatesd stop
Now you should be able to run Application -> Add/Remove.
Just remember to start the service after you are done.
You do this something like this, in a terminal.
$sudo service yum-updatesd
Alternatively, you could just reboot the machine.
-Manoj.C
