How can I run Unity in a virtual machine?
What do I need to download to run 12.04 LTS as a VM using VMware player v5.0 ?
You should download one of the 12.04 LTS .iso images, then tell VMware to mount those as the CD-ROM when booting.
Check Ubuntu help on installing Ubuntu inside VMware Player
Please refer section "Installing Ubuntu inside VMware Player on a non-Ubuntu OS" on the above mentioned page.