Ok, jsut had this same problem, and i solved it really easily!
the main.pac is on the CD right, so, I put the CD in my CDROM drive then edited the dosbox config file to look at it with this line:
"mount D I:\ -t cdrom"
because I:z is my CD drive, on your PC it's like... X or Y or something crazy
so jsut ahve a play and it should be sorted!
I only use dosbox for Z so my dosbox config file's configured for Z alone so when I load Dosbox i get Z.
# Lines in this section will be run at startup.
mount C C:\DOSGAME\Z
mount D I:\ -t cdrom
that's the end of my dosbox config file if it's any help.