Need help please! :)

just bought halo 4 and i installed disc two and then it says please insert halo 4 play disc how do i get on multiplayer?

The disk has to be in the reader to boot then it runs off the harddrive.

i already have disk 2 in though? so how do i play

Disc 2 is the installation disc. once your done installing, put in disc 1 and jump into multiplayer.