In order to take advantage of shared clipboard, seamless integration and drag&drop features of VirtualBox two things are needed. First, the guest additions need to be installed, pretty straight-forward in most distributions. Second,
VBoxClient needs to be started. This depends on desktop environment and distribution, some start it from the get-go, others don’t. If it’s started, it should show in the list of processes. If it isn’t, there’s a script that starts and enables all features, it’s called
VBoxClient-all. Just add that one to the startup list of the DE.