How to install UNIX In a virtual machine?

I got a laptop running Ubuntu 14.04 and I want to install some abandonware in virtual machines with VirtualBox and I chose to begin with AT&T UNIX 4 Version 2.1.I allocated for this machine 512 Mb of RAM,16 Gb storage,32 Mb video memory,1 CPU with 1.3 GHz and the boot order is CD/DVD;Floppy;Hard Disk.It says to press F12 to start the setup and afterwards it shows me VirtualBox temporary boot device selection,with four options: 1) Primary Master; f)floppy; c)CD-ROM; l)LAN; I tried all of them and and everytime it gives me the error:"FATAL Could not read from the boot medium.System halted." And my CD_ROM image is allright,I downloaded it from here and extracted it,then made an ISO file with Brasero,the file occupies now 44.3 Mb. What should I do? What did I do wrong?