[ubuntu] No bootable medium found

Complete virtualization newbie here.

I’m running an Acer Aspire One NetBook booting Ubuntu 12.4.1LTS 1.5Gib RAM A 32 bit 1.6GHz Intel Atom CPU with a 7.5Gb SSD. I added 128Gb SDHD chip for extra storage. No DVD on this Netbook
I installed VirtualBox and saved several ISOs on the 128Gb SDHD chip.

All of this is a recently new build. Everything is current.
Starting out easy, I wanted to build a 95, XP then 7 virtual OS to run one at a time.

However, when I try to build the virtual OS the progress bar doesn’t move and the system indicates 0% for an hour and a half. Drilling down, I find an error on a terminal screen saying ‘FATAL: No bootable medium found! System Halted.’

I tested this file by moving the SDHD chip to my desk top and burning a bootable DVD. I do not know what VB is looking for.

I don’t want to get hung up on any particular ISO, I just want to build a few virtual machines.

What do I need to do to get this working?

Please help!!