How do I remove an app installed if the buttons in "Manage Applications" are locked and can't be pressed?
Some apps register with the "Device Administrator" to avoid accidental/unwanted de-installation (e.g. child-protecting apps, so the child does not simply uninstall it -- also stuff which implements additional "rules", such as MS Exchange related apps). In this case, you need to remove them from the "Device Administrator" first:

Go to Settings&rarr;Security&rarr;Device Administrator and check if the app is listed there. If so, remove it from here. Now you should be able to uninstall the app.