i'm becoming really really frustrated with the following error message:

'the source can't be read.

maybe you don't have enough rights for this, or the source doesn't contain data (e.g: no disk in drive). (/dev)(/dvd)'

at first it thought it was a problem with the disk, so i changed it, but the message was the same. i switched DVD drives, but the error message was still the same.
i'm a complete n00b to ubuntu so i really have no idea what i'm doing, and ANY help would be muchly appreciated.