The VMX file is the disk image you're downloading. Its usually rarred or zipped. Such image is just an image of an OS you can load in a VM which is able to read its format. These images (sometimes called 'appliances') can be found everywhere on the Internet. The one for Maemo SDK for VMware (also works on VirtualBox OSE 1.6.3 on Linux/x86-64) can be found on earlier mentioned URL. I don't know if it will work with an other VirtualBox port or version. In theory, it should.