tags is ok, I'm assuming you are trying to execute [FILE]VBoxHeadless[/FILE] (the correct name).
All binaries are in [FILE]/usr/local/lib/virtualbox/[/FILE], symlinks are created under [FILE]/usr/local/bin[/FILE] though (so in the [FILE]PATH[/FILE] in standard setup).
I do use VirtualBox only in headless so I don't have X11/QT4 flags set; check with:
[CMD="#"]cd /usr/ports/emulators/virtualbox-ose
[/CMD]
[CMD="#"] make config
[/CMD]
I went briefly through [FILE]Makefile[/FILE] and it seems [FILE]--build-headless[/FILE] is only used when X11 is not selected. Please correct me if I'm wrong.
[CODE]
if ${PORT_OPTIONS:MX11}
USE_SDL= sdl
USE_XORG= xcursor xmu inputproto xinerama
VBOX_FRONTENDS+=VBoxBFE VBoxSDL
PLIST_SUB+= X11=""
.else
CONFIGURE_ARGS+=--build-headless
PLIST_SUB+= X11="@comment "
.endif