i recently bought pandora tomorrow (dvd) and installed it successfully. but when i started palying it, without the dvd, an error message which reads like "Please insert the correct CD" and when i put the cd back in, it worked. do i really need a cd to play or there's something wrong with the installation or what? thanks!

Yes, you do need the disc in to play.
By the way, if you try it, there are no servers anymore for the online portions of Pandora tomorrow. You will be confined to the single player mode.