Greenplum released Community Edition 4.1 which is a great free VM appliance you can run to get your feet wet with Greenplum and gain an understanding of what it can offer. Unfortunately it was only released to work on VMware Workstation or Fusion. Personally I like to run Parallels Desktop on my Macbook Air, so I worked to figure out how to make this work.
First off, the VM includes its vmdk broken up into 2GB chunks. Parallels may or may not be able to deal with this. For good measure, I just converted it to a single file. To do this I used VMware Fusion, unchecked the box to split the vmdk into 2GB chunks and let it do its thing. Another way you can accomplish this would be to use the QEMU tools:
qemu-img convert file.vmdk -O raw file.hdd
You can then take the file.hdd and add it to a VM you create in Parallels. Greenplum 4.1 CE does not require much resources, the default its only set for 1536MB and a single processor in the stock vmx file. So you could create a Parallels VM with a single processor, 1536MB memory and simply add the file.hdd as its hard disk.
When creating the VM in Parallels make sure you select IDE0:0 as the hard disk, it will fail otherwise.
If you try to import the VMX file into Parallels it may leave you with a CD-ROM trying to connect to an image that doesn’t exist as well, so you will need to correct that. There is really no benefit to importing the VMX. Best thing is to just create a new VM, as CentOS 4/5 64-bit, single processor, 1536MB memory and connect the raw .hdd file. Parallels should glue all of this together and successfully build a .pvm file.
You will want to install the Parallels Tools once the system boots.