Compile:

You need to have the VirtualBox libraries and headers around. Libraries are shipped with the VirtualBox binary package (and in ubuntu stored in /usr/lib/virtualbox). Header files are currently only found in the virtualbox source code, so at the moment you need to have the source code around. (cf. www.virtualbox.org) Pass both directories to configure with "--with-vbox-libdir=<path-to-libs>" and "with-vbox-incdir=<path-to-incs>".

Usage:
Use a kernel > 2.6.26! [Otherwise you'll need to use kpartx which is totally untested].
1) Load the nbd kernel module with max_part option (e.g. max_part=16).
2) Invoke VBoxMount as following: VBoxMount <nbd> <vdi> [vdi...] where <vdi> is your virtual disk image and <nbd> is any of /dev/nbdX where X is most likely something between 0 or 12.
  -> After that, you should have /dev/nbdXpY where Y is now mapped to the partitions.
3) You can mount those with the normal mount tools, e.g.: "mount /dev/nbd0p0 /mnt".

Unmounting:
"VBoxMount -u /dev/nbdX"

Have Fun.