Firstly, playback of DVDs is a paid feature. You can't play DVDs with the free version of RealPlayer.
Secondly, you will need to set your DVD Drive in Preferences. You can do so by checking under Preferences --> CD --> CD Burner
Lastly, if it is assigned to the correct drive and RealPlayer is not able to detect the disk, RealPlayer might not have installed correctly.
Please reinstall RealPlayer.