You need a copy of Fusion 3.1.3 and Pacifist 2.6.4, download them from:
http://downloads.vmware.com/d/info/desktop_end_user_computing/vmware_fusion/3_0
http://www.charlessoft.com
Drag the Pacifist application to your /Applications or /Utilities folder.
Instead of installing, mount the VMware-Fusion-3.1.3-416484.dmg disk image, right-click on the Install VMware Fusion application and click on Show Package Contents: then, in the Finder, navigate to Contents -> Resources -> Install VMware Fusion.mpkg, right-click on it and do an Open With -> Pacifist.
In Pacifist, navigate to Contents of Install VMware Fusion.mpkg -> Contents of Install Vmware Fusion.pkg -> Library -> Application Support -> VMware Fusion, then select only VMDKMounter.app and click the Install button in the Pacifist toolbar.
After the installation has completed, you will find the VMDKMounter application in the "/Library/Application Support/VMware Fusion" folder (but probably you can also extract it elsewhere, or move it afterwards; BTW, that folder isn't used by Fusion 4, anymore: everything now is inside the main application bundle).
The VMDKMounter now should work as usual (from the Finder context menu).
PS: MacFuse is just a driver to access 3rd party file systems build on top of OSX, but in any case you need to install it since VMDKMounter requires that. As MacFuse project page on Google tells that there is no further support/development. So, thats why VMware dropped it from latest version. This version of MacFuse works: http://www.tuxera.com/mac/macfuse/MacFUSE-Tuxera-2.2.dmg.
UPDATE: FUSE for OS X allows you to extend OS X's native file handling capabilities via third-party file systems. See answer by Dave_R for more details.